Streptomyces avermitilis gene directing the ratio of B2:B1 avermectins

ABSTRACT

The present invention relates to polynucleotide molecules comprising nucleotide sequences encoding an aveC gene product, which polynucleotide molecules can be used to alter the ratio or amount of class 2:1 avermectins produced in fermentation cultures of  S. avermitilis . The present invention further relates to vectors, host cells, and mutant strains of  S. avermitilis  in which the aveC gene has been inactivated, or mutated so as to change the ratio or amount of class 2:1 avermectins produced.

This application claims the benefit of Provisional Application60/356,222, filed Feb. 12, 2002.

1. FIELD OF THE INVENTION

The present invention is directed to compositions and methods for theefficient production of avermectins such as “doramectin”, which areprimarily useful in the field of animal health. More particularly, thepresent invention relates to polynucleotide molecules comprisingnucleotide sequences encoding an AveC gene product, which can be used tomodulate the ratio of class 2:1 avermectins produced by fermentationcultures of Streptomyces avermitilis. The present invention furtherrelates to vectors, transformed host cells, and novel mutant strains ofS. avermitilis in which the aveC gene has been mutated so as to modulatethe ratio of class 2:1 avermectins produced.

2. BACKGROUND OF THE INVENTION 2.1. Avermectins

Streptomyces species produce a wide variety of secondary metabolites,including the avermectins, which comprise a series of eight relatedsixteen-membered macrocyclic lactones having potent anthelmintic andinsecticidal activity. The eight distinct but closely related compoundsare referred to as A1a, A1b, A2a, A2b, B1a, B1b, B2a and B2b. The “a”series of compounds refers to the natural avermectin where thesubstituent at the C25 position is (S)-sec-butyl, and the “b” seriesrefers to those compounds where the substituent at the C25 position isisopropyl. The designations “A” and “B” refer to avermectins where thesubstituent at the C5 position is methoxy and hydroxy, respectively. Thenumeral “1” refers to avermectins where a double bond is present at theC22, 23 position, and the numeral “2” refers to avermectins having ahydrogen at the C22 position and a hydroxy at the C23 position. Amongthe related avermectins, the B1 type of avermectin, such as doramectin,is recognized as having the most effective antiparasitic and pesticidalactivity, and is therefore the most commercially desirable avermectin.

The avermectins and their production by aerobic fermentation of strainsof S. avermitilis are described in U.S. Pat. Nos. 4,310,519 and4,429,042. The biosynthesis of natural avermectins is believed to beinitiated endogenously from the CoA thioester analogs of isobutyric acidand S-(+)-2-methyl butyric acid.

A combination of both strain improvement through random mutagenesis andthe use of exogenously supplied fatty acids has led to the efficientproduction of avermectin analogs. Mutants of S. avermitilis that aredeficient in branched-chain 2-oxo acid dehydrogenase (bkd deficientmutants) can only produce avermectins when fermentations aresupplemented with fatty acids. Screening and isolation of mutantsdeficient in branched-chain dehydrogenase activity (e.g., S.avermitilis, ATCC 53567) are described in European Patent (EP) 276103.Fermentation of such mutants in the presence of exogenously suppliedfatty acids results in production of only the four avermectinscorresponding to the fatty acid employed. Thus, supplementingfermentations of S. avermitilis (ATCC 53567) with S-(+)-2-methylbutyricacid results in production of the natural avermectins A1a, A2a, B1a andB2a; supplementing fermentations with isobutyric acid results inproduction of the natural avermectins A1b, A2b, B1b, and B2b; andsupplementing fermentations with cyclopentanecarboxylic acid results inthe production of the four novel cyclopentylayermectins A1, A2, B1, andB2.

If supplemented with other fatty acids, novel avermectins are produced.By screening over 800 potential precursors, more than 60 other novelavermectins have been identified. (See, e.g., Dutton et al., 1991, J.Antibiot. 44:357-365; and Banks et al., 1994, Roy. Soc. Chem.147:16-26). In addition, mutants of S. avermitilis deficient in5-O-methyltransferase activity produce essentially only the B analogavermectins. Consequently, S. avermitilis mutants lacking bothbranched-chain 2-oxo acid dehydrogenase and 5-O-methyltransferaseactivity produce only the B avermectins corresponding to the fatty acidemployed to supplement the fermentation. Thus, supplementing such doublemutants with S-(+)-2-methylbutyric acid results in production of onlythe natural avermectins B1a and B2a, while supplementing with isobutyricacid or cyclopentanecarboxylic acid results in production of the naturalavermectins B1b and B2b or the novel cyclopentyl B1 and B2 avermectins,respectively. Supplementation of the double mutant strain withcyclohexane carboxylic acid is a preferred method for producing thecommercially important novel avermectin, cyclohexylayermectin B1(doramectin). The isolation and characteristics of such double mutants,e.g., S. avermitilis (ATCC 53692), is described in EP 276103.

2.2. Genes Involved in Avermectin Biosynthesis

In many cases, genes involved in production of secondary metabolites andgenes encoding a particular antibiotic are found clustered together onthe chromosome. Such is the case with the Streptomyces polyketidesynthase gene cluster (PKS) (see Hopwood and Sherman, 1990, Ann. Rev.Genet. 24:37-66). Thus, one strategy for cloning genes in a biosyntheticpathway has been to isolate a drug resistance gene and then testadjacent regions of the chromosome for other genes related to thebiosynthesis of that particular antibiotic. Another strategy for cloninggenes involved in the biosynthesis of important metabolites has beencomplementation of mutants. For example, portions of a DNA library froman organism capable of producing a particular metabolite are introducedinto a non-producing mutant and transformants screened for production ofthe metabolite. Additionally, hybridization of a library using probesderived from other Streptomyces species has been used to identify andclone genes in biosynthetic pathways.

Genes involved in avermectin biosynthesis (ave genes), like the genesrequired for biosynthesis of other Streptomyces secondary metabolites(e.g., PKS), are found clustered on the chromosome. A number of avegenes have been successfully cloned using vectors to complement S.avermitilis mutants blocked in avermectin biosynthesis. The cloning ofsuch genes is described in U.S. Pat. No. 5,252,474. In addition, Ikedaet al., 1995, J. Antibiot. 48:532-534, describes the localization of achromosomal region involving the C22,23 dehydration step (aveC) to a4.82 Kb BamHI fragment of S. avermitilis, as well as mutations in theaveC gene that result in the production of a single component B2aproducer. Since ivermectin, a potent anthelmintic compound, can beproduced chemically from avermectin B2a, such a single componentproducer of avermectin B2a is considered particularly useful forcommercial production of ivermectin.

U.S. Pat. No. 6,248,579 to Stutzman-Engwall et al., issued Jun. 19,2001, describes certain mutations to the aveC gene of Streptomycesavermitilis leading to a reduction in the ratio of cyclohexylB2:cyclohexyl B1 ratio to about 0.75:1.

PCT Publication WO 01/12821 by Pfizer Products Inc., published Feb. 22,2001, describes certain additional mutations to the aveC gene ofStreptomyces avermitilis leading to further reductions in the ratio ofcyclohexyl B2:cyclohexyl B1 ratio down to 0.40:1.

Identification of additional mutations or combinations of mutations inthe aveC gene that further minimize the complexity of avermectinproduction, such as, e.g., mutations that further decrease the B2:B1ratio of avermectins, would simplify production and purification ofcommercially important avermectins.

3. SUMMARY OF THE INVENTION

The present invention provides a polynucleotide molecule comprising anucleotide sequence that is otherwise the same as the Streptomycesavermitilis aveC allele, the S. avermitilis AveC gene product-encodingsequence of plasmid pSE186 (ATCC 209604) or the nucleotide sequence ofthe aveC ORF of S. avermitilis as presented in FIG. 1 (SEQ ID NO:1), ora degenerate variant thereof, but which nucleotide sequence furthercomprises mutations encoding a combination of amino acid substitutionsat amino acid residues corresponding to the amino acid positions of SEQID NO:2, such that cells of S. avermitilis strain ATCC 53692 in whichthe wild-type aveC allele has been inactivated and that express thepolynucleotide molecule comprising the mutated nucleotide sequence arecapable of producing a class 2:1 ratio of avermectins that is reducedcompared to the ratio produced by cells of S. avermitilis strain ATCC53692 that instead express only the wild-type aveC allele, wherein whenthe class 2:1 avermectins are cyclohexyl B2:cyclohexyl B1 avermectins,the ratio of class 2:1 avermectins is 0.35:1 or less.

In a more preferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.30:1 or less.

In a more preferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.25:1 or less.

In a more preferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.20:1 or less.

In a more preferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.10 or less.

In a particular embodiment thereof, the combination of amino acidsubstitutions comprises a combination selected from the group consistingof:

-   -   (a) D48E, A61T, A89T, S138T, A139T, G179S, A198G, P289L;    -   (b) G40S, D48E, L136P, G179S, E238D;    -   (c) D48E, L136P, R163Q, G179S;    -   (d) D48E, L136P, R163Q, G179S, E238D;    -   (e) D48E, L136P, R163Q, G179S, A200G, E238D;    -   (f) D48E, L136P, G179S, E238D;    -   (g) D48E, A61T, L136P, G179S, E238D;    -   (h) D48E, A61T, L136P, G179S;    -   (i) D48E, A89T, S138T, A139T, G179S;    -   (j) D48E, A61T, L136P, G179S, A198G, P202S, E238D, P289L;    -   (k) D48E, A61T, L136P, S138T, A139F, G179S, E238D, P289L;    -   (l) D48E, L136P, G179S, A198G, E238D, P289L;    -   (m) D48E, A61T, S138T, A139F, G179S, A198G, P289L;    -   (n) D48E, L84P, G111V, S138T, A139T, G179S, A198G, P289L;    -   (o) Y28C, D48E, A61T, A89T, S138T, A139T, G179S, E238D;    -   (p) D48E, A61T, A107T, S108G, L136P, G179S, S192A, E238D, P289L;    -   (q) D48E, L136P, G179S, R250W;    -   (r) D48E, A89T, S138T, A139T, R163Q, G179S;    -   (s) D48E, L136P, G179S, A198G, P289L;    -   (t) D48E, F78L, A89T, L136P, G179S;    -   (u) D48E, A89T, S138T, A139T, G179S, E238D, F278L;    -   (v) D48E, A89T, L136P, R163Q, G179S;    -   (w) D48E, A61T, A89T, G111V, S138T, A139F, G179S, E238D, P289L;    -   (x) D25G, D48E, A89T, L136P, S138T, A139T, V141A, 1159T, R163Q,        G179S;    -   (y) D48E, A89T, S90G, L136P, R163Q, G179S, E238D;    -   (z) D48E, A61T, A89T, G111V, S138T, A139T, G179S, E238D, P289L;    -   (aa) D48E, A89T, S138T, A139T, G179S;    -   (ab) D48E, L136P, R163Q, G179S, S231L;    -   (ac) D48E, L136P, S138T, A139F, G179S, V196A, E238D;    -   (ad) D48E, A61T, A89T, F99S, S138T, A139T, G179S, E238D;    -   (ae) G35S, D48E, A89T, S138T, A139T, G179S, P289L;    -   (af) D48E, A61T, A89T, S138T, A139T, G179S, V196A, E238D;    -   (ag) D48E, A89T, G111V, S138T, A139T, G179S, A198G, E238D;    -   (ah) S41G, D48E, A89T, L136P, G179S;    -   (ai) D48E, A89T, L136P, R163Q, G179S, P252S;    -   (aj) D48E, A89T, L136P, G179S, F234S;    -   (ak) D48E, A89T, L136P, R163Q, G179S, E238D;    -   (al) Q36R, D48E, A89T, L136P, G179S, E238D;    -   (am) D48E, A89T, L136P, R163Q, G179S;    -   (an) D48E, A89T, S138T, G179S;    -   (ao) D48E, A89T, L136P, G179S, E238D;    -   (ap) D48E, A89T, L136P, K154E, G179S, E238D;    -   (aq) D48E, A89T, S138T, A139T, K154R, G179S, V196A, P289L;    -   (ar) D48E, A89T, S138T, A139F, G179S, V196A, E238D;    -   (as) D48E, A61T, A89T, L136P, G179S, V196A, A198G, P289L;    -   (at) D48E, A61T, S138T, A139F, G179S, G196A, E238D, P289L;    -   (au) D48E, A89T, L136P, G179S;    -   (av) D48E, A89T, V120A, L136P, G179S;    -   (aw) D48E, A61T, A89T, S138T, A139F, G179S, V196A, A198G, E238D;    -   (ax) D48E, A61T, A89T, G111V, S138T, A139F, G179S, V196A, E238D;    -   (ay) D48E, A61T, A89T, S138T, A139T, G179S, V196A, E238D, P289L;    -   (az) D48E, A61T, A89T, L136P, S138T, A139F, G179S, A198G, E238D;    -   (ba) D48E, A89T, S138T, A139F, G179S, A198G, V220A;    -   (bb) D48E, A61T, A89T, S138T, A139T, G179S, V196A, E238D, R239H,        P289L;    -   (bc) D48E, A61T, A89T, L136P, G179S, P289L;    -   (bd) D48E, A89T, S138T, A139T, G179S, V196A, E238D, P289L;    -   (be) D48E, A61T, A89T, S138T, A139F, G179S, V196A, E238D;    -   (da) S41G, D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S,        G179S, V196A, E238D, F278L, P289L;    -   (db) S41G, D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S,        F176C, G179S, V196A, E238D, P289L;    -   (dc) D48E, R71L, A89T, L136P, T149S, F176C, G179S, E238D, I280V;    -   (dd) D48E, A61T, R71L, W110L, T149S, G179S, V196A, L206M, E238D,        V271A, I280V;    -   (de) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,        V196A, E238D, H279Q, P289L;    -   (df) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,        V196A, E238D, G287E, P289Q;    -   (dg) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,        V196A, E238D, P289L;    -   (dh) D48E, A61T, R71L, A89T, A139T, T149S, F176C, G179S, V196A,        E238D, V285G, P289L;    -   (di) Q38R, D48E, A61T, R71L, L87V, A89T, L136M, S138T, A139T,        T149S, G179S, V196A, E238D, P289L;    -   (dj) D48E, A61T, L87V, A89T, W110L, S138T, A139T, T149S, G179S,        V196A, E238D, P289L;    -   (dk) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,        V196A, E238D, P289L;    -   (dl) D48E, A89T, L136P, K154E, G179S, S231L, E238D;    -   (ea) D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S, V196A,        E238D, I280V;    -   (eb) D48E, R71L, A89T, L136P, T149S, F176C, G179S, E238D, I280V;    -   (ec) Q36P, D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S,        V196A, E238D, I280V;    -   (ed) D48E, A61T, R71L, A89T, A139T, T149S, F176C, G179S, V196A,        E238D, I280V;    -   (ee) V2M, D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S,        V196A, E238D, I280V;    -   (ef) D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S, V196A,        E238D, I280V, A302T;    -   (eg) D48E, R71L, A89T, L136P, T149S, F176C, G179S, E238D, P289L;    -   (eh) D48E, R71L, A89T, L136P, T149S, F176C, G179S, E238D, A302T;    -   (ei) D48E, R71L, A89T, L136P, T149S, F176C, G179S, V196A, E238D,        I280V;    -   (ej) D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S, V196A,        E238D;    -   (ek) V2M, D48E, R71L, A89T, L136P, T149S, F176C, G179S, V196A,        E238D, I280V;    -   (el) D48E, A61T, R71L, A89T, L136P, T149S, R162H, F176C, G179S,        V196A, E238D, I280V;    -   (em) D48E, R71L, A89T, V120A, L136P, T149S, K154E, G179S, S231L,        E238D;    -   (en) D48E, R71L, A89T, V120A, L136P, T149S, F176C, G179S, S231L,        E238D, I280V;    -   (eo) D48E, A61T, R71L, L87V, A89T, A139T, T149S, F176C, G179S,        V196A, E238D, V285G, P289L;    -   (ep) D48E, A61T, R71L, L87V, A89T, S90N, A139T, T149S, F176C,        G179S, V196A, E238D, V285G, P289L;    -   (eq) D48E, R71L, A89T, L136P, K154E, G179S, S231L, E238D;    -   (er) D48E, R71L, A89T, V120A, L136P, K154E, F176C, G179S, S231L,        E238D; and

(es) D48E, R71L, A89T, V120A, L136P, T149S, K154E, F176C, G179S, S231L,E238D.

The present invention further provides a polynucleotide moleculecomprising a nucleotide sequence that is otherwise the same as theStreptomyces avermitilis aveC allele, the S. avermitilis AveC geneproduct-encoding sequence of plasmid pSE186 (ATCC 209604) or thenucleotide sequence of the aveC ORF of S. avermitilis as presented inFIG. 1 (SEQ ID NO:1), or a degenerate variant thereof, but whichnucleotide sequence further comprises mutations encoding a combinationof amino acid substitutions at amino acid residues corresponding to theamino acid positions of SEQ ID NO:2, such that cells of S. avermitilisstrain ATCC 53692 in which the wild-type aveC allele has beeninactivated and that express a polynucleotide molecule comprising themutated nucleotide sequence are capable of producing a class 2:1 ratioof avermectins that is reduced compared to the ratio produced by cellsof S. avermitilis strain ATCC 53692 that instead express only thewild-type aveC allele, wherein when the class 2:1 avermectins arecyclohexyl B2:cyclohexyl B1 avermectins, the ratio of class 2:1avermectins is reduced to about 1.17:1 or less, and wherein thecombination of amino acid substitutions comprises a combination selectedfrom the group consisting of:

-   -   (bf) D48E, S138T, A139T, G179S, E238D;    -   (bg) Y28C, Q38R, D48E, L136P, G179S, E238D;    -   (ca) V3L, L136M;    -   (cb) G26A, D48Y, R75W, S93N;    -   (cc) R71 L;    -   (cd) T47I, W 110L, A139T;    -   (ce) V1041, S138T, V2201, F234I;    -   (cf) G45R, A64V, R69K;    -   (cg) S90N;    -   (ch) G26D, W110L, R233H;    -   (ci) Q36R, V1041, P128S, C152W, T276A;    -   (ck) C142Y, A302T;    -   (cl) V2M, V56D;    -   (cm) S41 G, L87V, A139T, L206M, G209R, I280V;    -   (cn) A62V, A139D;    -   (co) F176C;    -   (cp) T149S;    -   (cq) A64T, C142Y;    -   (cr) A130V, C142Y, L224M, E238V, L293M; and    -   (cs) A16T, K154M, L206F.

The present invention further provides a recombinant vector comprising apolynucleotide molecule of the present invention.

The present invention further provides a host cell comprising apolynucleotide molecule or a recombinant vector of the presentinvention. In a preferred embodiment, the host cell is a Streptomycescell. In a more preferred embodiment, the host cell is a cell ofStreptomyces avermitilis.

The present invention further provides a method for making a novelstrain of Streptomyces avermitilis, comprising (i) mutating the aveCallele in a cell of a strain of S. avermitilis, which mutation resultsin a combination of amino acid substitutions in the AveC gene product,or (ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein thecombination of amino acid substitutions is selected from (a) through(es) listed above.

The present invention further provides a method for making a novelstrain of Streptomyces avermitilis, comprising (i) mutating the aveCallele in a cell of a strain of S. avermitilis, which mutation resultsin a combination of amino acid substitutions in the AveC gene product,or (ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein cellscomprising the mutated aveC allele or degenerate variant are capable ofproducing cyclohexyl B2:cyclohexyl B1 avermectins in a ratio of 0.35:1or less. In a non-limiting embodiment thereof, the mutated aveC alleleor degenerate variant thereof encodes an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (a) through (be) and (da) through (es) listed above.

In a preferred embodiment thereof, the ratio of cyclohexyl B2:cyclohexylB1 avermectins is about 0.30:1 or less. In a non-limiting embodiment,the mutated aveC allele or degenerate variant thereof encodes an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (f) through (be) and (da) through(es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.25:1 or less. In a non-limitingembodiment, the mutated aveC allele or degenerate variant thereofencodes an AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (w) through (be) and(da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.20:1 or less. In a non-limitingembodiment, the mutated aveC allele or degenerate variant thereofencodes an AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (ao) through (be)and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.10:1 or less. In a non-limitingembodiment, the mutated aveC allele or degenerate variant thereofencodes an AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (ea) through (es)listed above.

The present invention further provides a method for making a novelstrain of Streptomyces avermitilis, comprising (i) mutating the aveCallele in a cell of a strain of S. avermitilis, which mutation resultsin a combination of amino acid substitutions in the AveC gene product,or (ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein thecombination of amino acid substitutions is selected from the groupconsisting of (bf),(bg) and (ca) through (cs) listed above.

In a preferred embodiment thereof, cells of S. avermitilis comprisingsuch a mutated aveC allele or degenerate variant are capable ofproducing cyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about1.17:1 or less.

The present invention further provides a cell of a Streptomyces speciesthat comprises a mutated S. avermitilis aveC allele or degeneratevariant thereof encoding an AveC gene product comprising a combinationof amino acid substitutions selected from (a) through (es) listed above.In a preferred embodiment thereof, the species of Streptomyces is S.avermitilis.

The present invention further provides a cell of Streptomycesavermitilis capable of producing cyclohexyl B2:cyclohexyl B1 avermectinsin a ratio of 0.35:1 or less. In a non-limiting embodiment thereof, thecell comprises a mutated aveC allele or degenerate variant thereofencoding an AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (a) through (be) and(da) through (es) listed above.

In a preferred embodiment thereof, the ratio of cyclohexyl B2:cyclohexylB1 avermectins is about 0.30:1 or less. In a non-limiting embodimentthereof, the cells comprise a mutated aveC allele or degenerate variantthereof encoding an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (f) through(be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.25:1 or less. In a non-limitingembodiment thereof, the cells comprise a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (w) through (be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.20:1 or less. In a non-limitingembodiment thereof, the cells comprise a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (ao) through (be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.10:1 or less. In a non-limitingembodiment thereof, the cells comprise a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (ea) through (es) listed above.

The present invention further provides a cell of a Streptomyces species,comprising a mutated S. avermitilis aveC allele or degenerate variantthereof encoding an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (bf), (bg), and(ca) through (cs) listed above. In a preferred embodiment thereof, thespecies of Streptomyces is S. avermitilis. In a more preferredembodiment, the cell is a cell of S. avermitilis capable of producingcyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 1.17:1 orless.

The present invention further provides a process for producingavermectins, comprising culturing a strain of Streptomyces avermitiliscells of the present invention in culture media under conditions thatpermit or induce the production of avermectins therefrom, and recoveringsaid avermectins from the culture.

The present invention further provides a composition of cyclohexylB2:cyclohexyl B1 avermectins produced by cells of Streptomycesavermitilis, comprising the cyclohexyl B2:cyclohexyl B1 avermectinspresent in a culture medium in which the cells have been cultured,wherein the ratio of the cyclohexyl B2:cyclohexyl B1 avermectins presentin the culture medium is 0.35:1 or less, preferably about 0.30:1 orless, more preferably about 0.25:1 or less, more preferably about 0.20:1or less and more preferably about 0.10:1 or less.

In a particular embodiment, the composition of cyclohexyl B2:cyclohexylB1 avermectins is produced by cells of a strain of S. avermitilis thatexpress a mutated aveC allele or degenerate variant thereof whichencodes a gene product that results in the reduction of the class 2:1ratio of cyclohexyl B2:cyclohexyl B1 avermectins produced by the cellscompared to cells of the same strain of S. avermitilis that do notexpress the mutated aveC allele but instead express only the wild-typeaveC allele.

In a preferred embodiment thereof, where the composition is cyclohexylB2:cyclohexyl B1 avermectins in a ratio of 0.35:1 or less, thecomposition is produced by cells comprising a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (a) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.30:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (f) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.25:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (w) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.20:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (ao) through (be) and (da) through (es) listedabove.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.10:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (ea) through (es) listed above

The present invention further provides a composition of cyclohexylB2:cyclohexyl B1 avermectins produced by cells of Streptomycesavermitilis, comprising the cyclohexyl B2:cyclohexyl B1 avermectinspresent in a culture medium in which the cells have been cultured,wherein the ratio of the cyclohexyl B2:cyclohexyl B1 avermectins presentin the culture medium is about 1.17:1 or less, and produced by cellscomprising a mutated aveC allele or degenerate variant thereof encodingan AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (bf) and (bg) and(ca) through (cs) listed above.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. DNA sequence (SEQ ID NO:1) comprising the S. avermitilis aveCORF, and deduced amino acid sequence (SEQ ID NO:2).

FIG. 2. Plasmid vector pSE186 (ATCC 209604) comprising the entire ORF ofthe aveC gene of S. avermitilis.

FIG. 3. Gene replacement vector pSE180 (ATCC 209605) comprising the ermEgene of Sacc. erythraea inserted into the aveC ORF of S. avermitilis.

FIG. 4. BamHI restriction map of the avermectin polyketide synthase genecluster from S. avermitilis with five overlapping cosmid clonesidentified (i.e., pSE65, pSE66, pSE67, pSE68, pSE69). The relationshipof pSE118 and pSE119 is also indicated.

FIG. 5. HPLC analysis of fermentation products produced by S.avermitilis strains. Peak quantitation was performed by comparison tostandard quantities of cyclohexyl B1. Cyclohexyl B2 retention time was7.4-7.7 min; cyclohexyl B1 retention time was 11.9-12.3 min. FIG. 5A. S.avermitilis strain SE180-11 with an inactivated aveC ORF. FIG. 5B. S.avermitilis strain SE180-11 transformed with pSE186 (ATCC 209604). FIG.5C. S. avermitilis strain SE180-11 transformed with pSE187. FIG. 5D. S.avermitilis strain SE180-11 transformed with pSE188.

FIGS. 6A-M. Compiled list of combinations of amino acid substitutionsencoded by mutations to the aveC allele as identified by a second roundof “gene shuffling”, and their effects on the ratio of cyclohexylB2:cyclohexyl B1 production. For each plasmid, in the column entitled“Mutations”, the upper box lists the amino acid substitutions, and thelower box lists the nucleotide base changes resulting in those aminoacid substitutions. Nucleotide base changes in parentheses are silentchanges, i.e., they do not result in changes to the amino acid sequence.

The present invention relates to the identification and characterizationof polynucleotide molecules having nucleotide sequences that encode theAveC gene product from Streptomyces avermitilis, the construction ofnovel strains of S. avermitilis that can be used to screen mutated AveCgene products for their effect on avermectin production, and thediscovery that certain mutated AveC gene products can reduce the ratioof B2:B1 avermectins produced by S. avermitilis. By way of example, theinvention is described in the sections below for a polynucleotidemolecule having either a nucleotide sequence that is the same as the S.avermitilis AveC gene product-encoding sequence of plasmid pSE186 (ATCC209604), or the nucleotide sequence of the ORF of FIG. 1 (SEQ ID NO:1),and for polynucleotides molecules having mutated nucleotide sequencesderived therefrom and degenerate variants thereof. However, theprinciples set forth in the present invention can be analogously appliedto other polynucleotide molecules, including aveC homolog genes fromother Streptomyces species including, e.g., S. hygroscopicus and S.griseochromogenes, among others.

5.1. Polynucleotide Molecules Encoding

The S. avermitilis AveC Gene Product The present invention provides anisolated polynucleotide molecule comprising the complete aveC ORF of S.avermitilis or a substantial portion thereof, which isolatedpolynucleotide molecule lacks the next complete ORF that is locateddownstream from the aveC ORF in situ in the S. avermitilis chromosome.

The isolated polynucleotide molecule of the present invention preferablycomprises a nucleotide sequence that is the same as the S. avermitilisAveC gene product-encoding sequence of plasmid pSE186 (ATCC 209604), orthat is the same as the nucleotide sequence of the ORF of FIG. 1 (SEQ IDNO:1) or substantial portion thereof. As used herein, a “substantialportion” of an isolated polynucleotide molecule comprising a nucleotidesequence encoding the S. avermitilis AveC gene product means an isolatedpolynucleotide molecule comprising at least about 70% of the completeaveC ORF sequence shown in FIG. 1 (SEQ ID NO:1), and that encodes afunctionally equivalent AveC gene product. In this regard, a“functionally equivalent” AveC gene product is defined as a gene productthat, when expressed in S. avermitilis strain ATCC 53692 in which thenative aveC allele has been inactivated, results in the production ofsubstantially the same ratio and amount of avermectins as produced by S.avermitilis strain ATCC 53692 which instead expresses only thewild-type, functional aveC allele native to S. avermitilis strain ATCC53692.

In addition to the nucleotide sequence of the aveC ORF, the isolatedpolynucleotide molecule of the present invention can further comprisenucleotide sequences that naturally flank the aveC gene in situ in S.avermitilis, such as those flanking nucleotide sequences shown in FIG. 1(SEQ ID NO:1).

The present invention further provides an isolated polynucleotidemolecule comprising the nucleotide sequence of SEQ ID NO:1 or adegenerate variant thereof, as based on the known degeneracy of thegenetic code.

As used herein, the terms “polynucleotide molecule,” “polynucleotidesequence,” “coding sequence,” “open-reading frame,” and “ORF” areintended to refer to both DNA and RNA molecules, which can either besingle-stranded or double-stranded, and that can be transcribed andtranslated (DNA), or translated (RNA), into an AveC gene product, orinto a polypeptide that is homologous to an AveC gene product in anappropriate host cell expression system when placed under the control ofappropriate regulatory elements. A coding sequence can include but isnot limited to prokaryotic sequences, cDNA sequences, genomic DNAsequences, and chemically synthesized DNA and RNA sequences.

The nucleotide sequence shown in FIG. 1 (SEQ ID NO:1) comprises fourdifferent GTG codons at bp positions 42, 174, 177 and 180. As previouslydescribed in U.S. Pat. No. 6,248,579, multiple deletions of the 5′region of the aveC ORF (FIG. 1; SEQ ID NO:1) were constructed to helpdefine which of these codons could function in the aveC ORF as startsites for protein expression. Deletion of the first GTG site at bp 42did not eliminate AveC activity. Additional deletion of all of the GTGcodons at bp positions 174, 177 and 180 together eliminated AveCactivity, indicating that this region is necessary for proteinexpression. The present invention thus encompasses variable length aveCORFs.

The present invention further provides a polynucleotide molecule havinga nucleotide sequence that is homologous to the S. avermitilis AveC geneproduct-encoding sequence of plasmid pSE186 (ATCC 209604), or to thenucleotide sequence of the aveC ORF presented in FIG. 1 (SEQ ID NO:1) orsubstantial portion thereof. The term “homologous” when used to refer toa polynucleotide molecule that is homologous to an S. avermitilis AveCgene product-encoding sequence means a polynucleotide molecule having anucleotide sequence: (a) that encodes the same AveC gene product as theS. avermitilis AveC gene product-encoding sequence of plasmid pSE186(ATCC 209604), or that encodes the same AveC gene product as thenucleotide sequence of the aveC ORF presented in FIG. 1 (SEQ ID NO:1),but that includes one or more silent changes to the nucleotide sequenceaccording to the degeneracy of the genetic code (i.e., a degeneratevariant); or (b) that hybridizes to the complement of a polynucleotidemolecule having a nucleotide sequence that encodes the amino acidsequence encoded by the AveC gene product-encoding sequence of plasmidpSE186 (ATCC 209604) or that encodes the amino acid sequence shown inFIG. 1 (SEQ ID NO:2) under moderately stringent conditions, i.e.,hybridization to filter-bound DNA in 0.5 M NaHPO₄, 7% sodium dodecylsulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.2×SSC/0.1% SDS at42° C. (see Ausubel et al. (eds.), 1989, Current Protocols in MolecularBiology, Vol. I, Green Publishing Associates, Inc., and John Wiley &Sons, Inc., New York, at p. 2.10.3), and encodes a functionallyequivalent AveC gene product as defined above. In a preferredembodiment, the homologous polynucleotide molecule hybridizes to thecomplement of the AveC gene product-encoding nucleotide sequence ofplasmid pSE186 (ATCC 209604) or to the complement of the nucleotidesequence of the aveC ORF presented in FIG. 1 (SEQ ID NO:1) orsubstantial portion thereof under highly stringent conditions, i.e.,hybridization to filter-bound DNA in 0.5 M NaHPO₄, 7% SDS, 1 mM EDTA at65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. (Ausubel et al., 1989,above), and encodes a functionally equivalent AveC gene product asdefined above.

The activity of an AveC gene product and potential functionalequivalents thereof can be determined through HPLC analysis offermentation products, as described in the examples below.Polynucleotide molecules having nucleotide sequences that encodefunctional equivalents of the S. avermitilis AveC gene product mayinclude naturally occurring aveC genes present in other strains of S.avermitilis, aveC homolog genes present in other species ofStreptomyces, and mutated aveC alleles, whether naturally occurring orengineered.

The present invention further provides a polynucleotide moleculecomprising a nucleotide sequence that encodes a polypeptide having anamino acid sequence that is homologous to the amino acid sequenceencoded by the AveC gene product-encoding sequence of plasmid pSE186(ATCC 209604), or the amino acid sequence of FIG. 1 (SEQ ID NO:2) orsubstantial portion thereof. As used herein, a “substantial portion” ofthe amino acid sequence of FIG. 1 (SEQ ID NO:2) means a polypeptidecomprising at least about 70% of the amino acid sequence shown in FIG. 1(SEQ ID NO:2), and that constitutes a functionally equivalent AveC geneproduct, as defined above.

As used herein to refer to amino acid sequences that are homologous tothe amino acid sequence of an AveC gene product from S. avermitilis, theterm “homologous” refers to a polypeptide which otherwise has the aminoacid sequence of FIG. 1 (SEQ ID NO:2), but in which one or more aminoacid residues has been conservatively substituted with a different aminoacid residue, wherein said amino acid sequence has at least about 70%,more preferably at least about 80%, and most preferably at least about90% amino acid sequence identity to the polypeptide encoded by the AveCgene product-encoding sequence of plasmid pSE186 (ATCC 209604) or theamino acid sequence of FIG. 1 (SEQ ID NO:2) as determined by anystandard amino acid sequence identity algorithm, such as the BLASTPalgorithm (GENBANK, NCBI), and where such conservative substitutionresults in a functionally equivalent gene product, as defined above.Conservative amino acid substitutions are well known in the art. Rulesfor making such substitutions include those described by Dayhof, M.D.,1978, Nat. Biomed. Res. Found., Washington, D.C., Vol. 5, Sup. 3, amongothers. More specifically, conservative amino acid substitutions arethose that generally take place within a family of amino acids that arerelated in the acidity or polarity. Genetically encoded amino acids aregenerally divided into four groups: (1) acidic=aspartate, glutamate; (2)basic=lysine, arginine, histidine; (3) non-polar=alanine, valine,leucine, isoleucine, proline, phenylalanine, methionine, tryptophan; and(4) uncharged polar=glycine, asparagine, glutamine, cysteine, serine,threonine, tyrosine. Phenylalanine, tryptophan and tyrosine are alsojointly classified as aromatic amino acids. One or more replacementswithin any particular group, e.g., of a leucine with an isoleucine orvaline, or of an aspartate with a glutamate, or of a threonine with aserine, or of any other amino acid residue with a structurally relatedamino acid residue, e.g., an amino acid residue with similar acidity orpolarity, or with similarity in some combination thereof, will generallyhave an insignificant effect on the function of the polypeptide.

Production and manipulation of the polynucleotide molecules disclosedherein are within the skill in the art and can be carried out accordingto recombinant techniques described, e.g., in Maniatis, et al., 1989,Molecular Cloning, A Laboratory Manual, Cold Spring Harbor LaboratoryPress, Cold Spring Harbor, N.Y.; Ausubel, et al., 1989, CurrentProtocols In Molecular Biology, Greene Publishing Associates & WileyInterscience, NY; Sambrook, et al., 1989, Molecular Cloning: ALaboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, ColdSpring Harbor, N.Y.; Innis et al. (eds), 1995, PCR Strategies, AcademicPress, Inc., San Diego; and Erlich (ed), 1992, PCR Technology, OxfordUniversity Press, New York, all of which are incorporated herein byreference. Polynucleotide clones encoding AveC gene products or AveChomolog gene products can be identified using any method known in theart, including but not limited to the methods set forth in Section 7,below. Genomic DNA libraries can be screened for aveC and aveC homologcoding sequences using techniques such as the methods set forth inBenton and Davis, 1977, Science 196:180, for bacteriophage libraries,and in Grunstein and Hogness, 1975, Proc. Natl. Acad. Sci. USA,72:3961-3965, for plasmid libraries. Polynucleotide molecules havingnucleotide sequences known to include the aveC ORF, as present, e.g., inplasmid pSE186 (ATCC 209604), or in plasmid pSE119 (described in Section7, below), can be used as probes in these screening experiments.Alternatively, oligonucleotide probes can be synthesized that correspondto nucleotide sequences deduced from partial or complete amino acidsequences of the purified AveC homolog gene product.

5.2. Recombinant Systems 5.2.1. Cloning and Expression Vectors

The present invention further provides recombinant cloning vectors andexpression vectors which are useful in cloning or expressingpolynucleotide molecules of the present invention comprising, e.g., theaveC ORF of S. avermitilis or any aveC homolog ORFs. In a non-limitingembodiment, the present invention provides plasmid pSE186 (ATCC 209604),which comprises the complete ORF of the aveC gene of S. avermitilis.

All of the following description regarding the aveC ORF from S.avermitilis, or a polynucleotide molecule comprising the aveC ORF fromS. avermitilis or portion thereof, or an S. avermitilis AveC geneproduct, also refers to mutated aveC alleles as described below, unlessindicated explicitly or by context.

A variety of different vectors have been developed for specific use inStreptomyces, including phage, high copy number plasmids, low copynumber plasmids, and E. coli-Streptomyces shuttle vectors, among others,and any of these can be used to practice the present invention. A numberof drug resistance genes have also been cloned from Streptomyces, andseveral of these genes have been incorporated into vectors as selectablemarkers. Examples of current vectors for use in Streptomyces arepresented, among other places, in Hutchinson, 1980, Applied Biochem.Biotech. 16:169-190.

Recombinant vectors of the present invention, particularly expressionvectors, are preferably constructed so that the coding sequence for thepolynucleotide molecule of the invention is in operative associationwith one or more regulatory elements necessary for transcription andtranslation of the coding sequence to produce a polypeptide. As usedherein, the term “regulatory element” includes but is not limited tonucleotide sequences that encode inducible and non-inducible promoters,enhancers, operators and other elements known in the art that serve todrive and/or regulate expression of polynucleotide coding sequences.Also, as used herein, the coding sequence is in “operative association”with one or more regulatory elements where the regulatory elementseffectively regulate and allow for the transcription of the codingsequence or the translation of its mRNA, or both.

Typical plasmid vectors that can be engineered to contain apolynucleotide molecule of the present invention include pCR-Blunt,pCR2.1 (Invitrogen), pGEM3Zf (Promega), and the shuttle vector pWHM3(Vara et al., 1989, J. Bact. 171:5872-5881), among many others.

Methods are well-known in the art for constructing recombinant vectorscontaining particular coding sequences in operative association withappropriate regulatory elements, and these can be used to practice thepresent invention. These methods include in vitro recombinanttechniques, synthetic techniques, and in vivo genetic recombination.See, e.g., the techniques described in Maniatis et al., 1989, above;Ausubel et al., 1989, above; Sambrook et al., 1989, above; Innis et al.,1995, above; and Erlich, 1992, above.

The regulatory elements of these vectors can vary in their strength andspecificities. Depending on the host/vector system utilized, any of anumber of suitable transcription and translation elements can be used.Non-limiting examples of transcriptional regulatory regions or promotersfor bacteria include the β-gal promoter, the T7 promoter, the TACpromoter, λ left and right promoters, trp and lac promoters, trp-lacfusion promoters and, more specifically for Streptomyces, the promotersermE, meiC, and tipA, etc. In a specific embodiment, an expressionvector can be generated that contains the aveC ORF or mutated ORFthereof cloned adjacent to a strong constitutive promoter, such as theermE promoter from Saccharopolyspora erythraea. As described in U.S.Pat. No. 6,248,579, a vector comprising the ermE promoter wastransformed into S. avermitilis, and subsequent HPLC analysis offermentation products indicated an increased titer of avermectinsproduced compared to production by the same strain which insteadexpresses only the wild-type aveC allele.

Fusion protein expression vectors can be used to express an AveC geneproduct-fusion protein. The purified fusion protein can be used to raiseantisera against the AveC gene product, to study the biochemicalproperties of the AveC gene product, to engineer AveC fusion proteinswith different biochemical activities, or to aid in the identificationor purification of the expressed AveC gene product. Possible fusionprotein expression vectors include but are not limited to vectorsincorporating sequences that encode β-galactosidase and trpE fusions,maltose-binding protein fusions, glutathione-S-transferase fusions andpolyhistidine fusions (carrier regions). In an alternative embodiment,an AveC gene product or a portion thereof can be fused to an AveChomolog gene product, or portion thereof, derived from another speciesor strain of Streptomyces, such as, e.g., S. hygroscopicus or S.griseochromogenes. Such hybrid vectors can be transformed into S.avermitilis cells and tested to determine their effect, e.g., on theratio of class 2:1 avermectin produced.

AveC fusion proteins can be engineered to comprise a region useful forpurification. For example, AveC-maltose-binding protein fusions can bepurified using amylose resin; AveC-glutathione-S-transferase fusionproteins can be purified using glutathione-agarose beads; andAveC-polyhistidine fusions can be purified using divalent nickel resin.Alternatively, antibodies against a carrier protein or peptide can beused for affinity chromatography purification of the fusion protein. Forexample, a nucleotide sequence coding for the target epitope of amonoclonal antibody can be engineered into the expression vector inoperative association with the regulatory elements and situated so thatthe expressed epitope is fused to the AveC polypeptide. For example, anucleotide sequence coding for the FLAG™ epitope tag (InternationalBiotechnologies Inc.), which is a hydrophilic marker peptide, an beinserted by standard techniques into the expression vector at a pointcorresponding, e.g., to the carboxyl terminus of the AveC polypeptide.The expressed AveC polypeptide-FLAG™ epitope fusion product can then bedetected and affinity-purified using commercially available anti-FLAG™antibodies.

The expression vector encoding the AveC fusion protein can also beengineered to contain polylinker sequences that encode specific proteasecleavage sites so that the expressed AveC polypeptide can be releasedfrom the carrier region or fusion partner by treatment with a specificprotease. For example, the fusion protein vector can include DNAsequences encoding thrombin or factor Xa cleavage sites, among others.

A signal sequence upstream from, and in reading frame with, the aveC ORFcan be engineered into the expression vector by known methods to directthe trafficking and secretion of the expressed gene product.Non-limiting examples of signal sequences include those from α-factor,immunoglobulins, outer membrane proteins, penicillinase, and T-cellreceptors, among others.

To aid in the selection of host cells transformed or transfected withcloning or expression vectors of the present invention, the vector canbe engineered to further comprise a coding sequence for a reporter geneproduct or other selectable marker. Such a coding sequence is preferablyin operative association with the regulatory element coding sequences,as described above. Reporter genes that are useful in the invention arewell-known in the art and include those encoding green fluorescentprotein, luciferase, xylE, and tyrosinase, among others. Nucleotidesequences encoding selectable markers are well known in the art, andinclude those that encode gene products conferring resistance toantibiotics or anti-metabolites, or that supply an auxotrophicrequirement. Examples of such sequences include those that encoderesistance to erythromycin, thiostrepton or kanamycin, among manyothers.

5.2.2. Transformation of Host Cells

The present invention further provides transformed host cells comprisinga polynucleotide molecule or recombinant vector of the invention, andnovel strains or cell lines derived therefrom. Host cells useful in thepractice of the invention are preferably Streptomyces cells, althoughother prokaryotic cells or eukaryotic cells can also be used. Suchtransformed host cells typically include but are not limited tomicroorganisms, such as bacteria transformed with recombinantbacteriophage DNA, plasmid DNA or cosmid DNA vectors, or yeasttransformed with recombinant vectors, among others.

The polynucleotide molecules of the present invention are intended tofunction in Streptomyces cells, but can also be transformed into otherbacterial or eukaryotic cells, e.g., for cloning or expression purposes.A strain of E. coli can typically be used, such as, e.g., the DH5αstrain, available from the American Type Culture Collection (ATCC),Rockville, Md., USA (Accession No. 31343), and from commercial sources(Stratagene). Preferred eukaryotic host cells include yeast cells,although mammalian cells or insect cells can also be utilizedeffectively.

The recombinant expression vector of the invention is preferablyintroduced, e.g., transformed or transfected, into one or more hostcells of a substantially homogeneous culture of cells. The expressionvector is generally introduced into host cells in accordance with knowntechniques, such as, e.g., by protoplast transformation, calciumphosphate precipitation, calcium chloride treatment, microinjection,electroporation, transfection by contact with a recombined virus,liposome-mediated transfection, DEAE-dextran transfection, transduction,conjugation, or microprojectile bombardment. Selection of transformantscan be conducted by standard procedures, such as by selecting for cellsexpressing a selectable marker, e.g., antibiotic resistance, associatedwith the recombinant vector, as described above.

Once the expression vector is introduced into the host cell, theintegration and maintenance of the aveC coding sequence either in thehost cell chromosome or episomally can be confirmed by standardtechniques, e.g., by Southern hybridization analysis, restriction enzymeanalysis, PCR analysis, including reverse transcriptase PCR (rt-PCR), orby immunological assay to detect the expected gene product. Host cellscontaining and/or expressing the recombinant aveC coding sequence can beidentified by any of at least four general approaches which arewell-known in the art, including: (i) DNA-DNA, DNA-RNA, or RNA-antisenseRNA hybridization; (ii) detecting the presence of “marker” genefunctions; (iii) assessing the level of transcription as measured by theexpression of aveC-specific mRNA transcripts in the host cell; and (iv)detecting the presence of mature polypeptide product as measured, e.g.,by immunoassay or by the presence of AveC biological activity (e.g., theproduction of specific ratios and amounts of avermectins indicative ofAveC activity in, e.g., S. avermitilis host cells).

5.2.3. Expression and Characterization of a Recombinant AveC GeneProduct

Once the native or mutated aveC coding sequence has been stablyintroduced into an appropriate host cell, the transformed host cell isclonally propagated, and the resulting cells can be grown underconditions conducive to the maximum production of the native or mutatedAveC gene product. Such conditions typically include growing cells tohigh density. Where the expression vector comprises an induciblepromoter, appropriate induction conditions such as, e.g., temperatureshift, exhaustion of nutrients, addition of gratuitous inducers (e.g.,analogs of carbohydrates, such as isopropyl-β-D-thiogalactopyranoside(IPTG)), accumulation of excess metabolic by-products, or the like, areemployed as needed to induce expression.

Where the expressed AveC gene product is retained inside the host cells,the cells are harvested and lysed, and the product isolated and purifiedfrom the lysate under extraction conditions known in the art to minimizeprotein degradation such as, e.g., at 4° C., or in the presence ofprotease inhibitors, or both. Where the expressed AveC gene product issecreted from the host cells, the exhausted nutrient medium can simplybe collected and the product isolated therefrom.

The expressed AveC gene product can be isolated or substantiallypurified from cell lysates or culture medium, as appropriate, usingstandard methods, including but not limited to any combination of thefollowing methods: ammonium sulfate precipitation, size fractionation,ion exchange chromatography, HPLC, density centrifugation, and affinitychromatography. Where the expressed AveC gene product exhibitsbiological activity, increasing purity of the preparation can bemonitored at each step of the purification procedure by use of anappropriate assay. Whether or not the expressed AveC gene productexhibits biological activity, it can be detected as based, e.g., onsize, or reactivity with an antibody otherwise specific for AveC, or bythe presence of a fusion tag. As used herein, an AveC gene product is“substantially purified” where the product constitutes more than about20 wt % of the protein in a particular preparation. Also, as usedherein, an AveC gene product is “isolated” where the product constitutesat least about 80 wt % of the protein in a particular preparation.

The present invention thus provides a recombinantly-expressed isolatedor substantially purified S. avermitilis AveC gene product comprisingthe amino acid sequence encoded by the AveC gene product-encodingsequence of plasmid pSE186 (ATCC 209604), or the amino acid sequence ofFIG. 1 (SEQ ID NO:2) or a substantial portion thereof, and mutatedversions and degenerate variants thereof.

The present invention further provides a method for producing an AveCgene product, comprising culturing a host cell transformed with arecombinant expression vector, said vector comprising a polynucleotidemolecule having a nucleotide sequence encoding the AveC gene product,which polynucleotide molecule is in operative association with one ormore regulatory elements that control expression of the polynucleotidemolecule in the host cell, under conditions conducive to the productionof the recombinant AveC gene product, and recovering the AveC geneproduct from the cell culture.

The recombinantly expressed S. avermitilis AveC gene product is usefulfor a variety of purposes, including for screening compounds that alterAveC gene product function and thereby modulate avermectin biosynthesis,and for raising antibodies directed against the AveC gene product.

Once an AveC gene product of sufficient purity has been obtained, it canbe characterized by standard methods, including by SDS-PAGE, sizeexclusion chromatography, amino acid sequence analysis, biologicalactivity in producing appropriate products in the avermectinbiosynthetic pathway, etc. For example, the amino acid sequence of theAveC gene product can be determined using standard peptide sequencingtechniques. The AveC gene product can be further characterized usinghydrophilicity analysis (see, e.g., Hopp and Woods, 1981, Proc. Natl.Acad. Sci. USA 78:3824), or analogous software algorithms, to identifyhydrophobic and hydrophilic regions of the AveC gene product. Structuralanalysis can be carried out to identify regions of the AveC gene productthat assume specific secondary structures. Biophysical methods such asX-ray crystallography (Engstrom, 1974, Biochem. Exp. Biol. 11: 7-13),computer modelling (Fletterick and Zoller (eds), 1986, in: CurrentCommunications in Molecular Biology, Cold Spring Harbor Laboratory, ColdSpring Harbor, N.Y.), and nuclear magnetic resonance (NMR) can be usedto map and study sites of interaction between the AveC gene product andits substrate. Information obtained from these studies can be used toselect new sites for mutation in the aveC ORF to help develop newstrains of S. avermitilis having more desirable avermectin productioncharacteristics.

5.3. Construction and Use of AveC Mutants

A primary objective of the present invention is to identify novelmutations in the aveC allele of S. avermitilis that result in a change,and most preferably a reduction, in the ratio of B2:B1 avermectins. Thepresent invention thus provides polynucleotide molecules useful toproduce novel strains of S. avermitilis cells that exhibit a detectablechange in avermectin production compared to cells of the same strain butwhich instead express only the wild-type aveC allele. In a preferredembodiment, such polynucleotide molecules are useful to produce novelstrains of S. avermitilis cells that produce avermectins in a reducedclass 2:1 ratio compared to cells of the same strain which insteadexpress only the wild-type aveC allele. The cells of such strains canalso comprise additional mutations to produce an increased amount ofavermectins compared to cells of the same strain that instead expressonly a single wild-type aveC allele.

Mutations to the aveC allele or coding sequence include any mutationsthat introduce one or more amino acid substitutions, deletions and/oradditions into the AveC gene product, or that result in truncation ofthe AveC gene product, or any combination thereof, and that produce thedesired result. Such mutated aveC allele sequences are intended toinclude any degenerate variants thereof. For example, the presentinvention provides a polynucleotide molecule comprising the nucleotidesequence of the aveC allele or a degenerate variant thereof, or the AveCgene product-encoding sequence of plasmid pSE186 (ATCC 209604) or adegenerate variant thereof, or the nucleotide sequence of the aveC ORFof S. avermitilis as present in FIG. 1 (SEQ ID NO:1) or a degeneratevariant thereof, but that further comprises mutations that encode acombination of amino acid substitutions at selected positions in theAveC gene product.

In a non-limiting embodiment, such substitutions occur at one or moreamino acid positions of the AveC gene product corresponding to aminoacid positions 2, 25, 28, 35, 36, 38, 40, 41, 48, 55, 61, 71, 78, 84,89, 90, 99, 107, 108, 111, 120, 123, 136, 138, 139, 141, 149, 154, 159,162, 163, 176, 179, 192, 196, 198, 200, 202, 220, 228, 229, 230, 231,234, 238, 239, 250, 252, 266, 275, 278, 280, 289 or 298 of SEQ ID NO:2.Preferred combinations of amino acid positions to be substitutedcomprise one or more of amino acid residues D48, A61, R71, A89, L136,S138, A139, T149, R163, F176, G179, V196, A198, E238 and P289.

Specifically preferred combinations of amino acid substitutions comprisesubstitutions at both D48 and G179, and more specifically D48E andG179S. Specific examples of combinations of amino acid substitutionsthat result in a reduction in cyclohexyl B:cyclohexyl B1 ratios arelisted in FIGS. 6A-M.

The present invention thus provides a polynucleotide molecule comprisinga nucleotide sequence that is otherwise the same as the Streptomycesavermitilis aveC allele, the S. avermitilis AveC gene product-encodingsequence of plasmid pSE186 (ATCC 209604) or the nucleotide sequence ofthe aveC ORF of S. avermitilis as presented in FIG. 1 (SEQ ID NO:1), ora degenerate variant thereof, but which nucleotide sequence furthercomprises mutations encoding a combination of amino acid substitutionsat amino acid residues corresponding to the amino acid positions of SEQID NO:2, such that cells of S. avermitilis strain ATCC 53692 in whichthe wild-type aveC allele has been inactivated and that express thepolynucleotide molecule comprising the mutated nucleotide sequence arecapable of producing a class 2:1 ratio of avermectins that is reducedcompared to the ratio produced by cells of S. avermitilis strain ATCC53692 that instead express only the wild-type aveC allele, wherein whenthe class 2:1 avermectins are cyclohexyl B2:cyclohexyl B1 avermectins,the ratio of class 2:1 avermectins is 0.35:1 or less. In a morepreferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.30:1 or less. In a more preferred embodiment, theratio of cyclohexyl B2:cyclohexyl B1 avermectins is about 0.25:1 orless. In a more preferred embodiment, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.20:1 or less. In a morepreferred embodiment, the ratio of cyclohexyl B2:cyclohexyl B1avermectins is about 0.10:1 or less

In a particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (a): D48E, A61T, A89T,S138T, A139T, G179S, A198G, P289L. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE538 (see FIG. 6).

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (b): G40S, D48E, L136P,G179S, E238D. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE559.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (c): D48E, L136P,R163Q, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE567.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (d): D48E, L136P,R163Q, G179S, E238D. Non-limiting examples of plasmids encoding theseamino acid substitutions are pSE570 and pSE572.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (e): D48E, L136P,R163Q, G179S, A200G, E238D. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE571.

In a particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (f): D48E, L136P,G179S, E238D. Non-limiting examples of plasmids encoding these aminoacid substitutions are pSE501 and pSE546.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (g): D48E, A61T, L136P,G179S, E238D. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE510.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (h): D48E, A61T, L136P,G179S. A non-limiting example of a plasmid encoding these amino acidsubstitutions is pSE512.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (i): D48E, A89T, S138T,A139T, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE519.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (j): D48E, A61T, L136P,G179S, A198G, P202S, E238D, P289L. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE526.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (k): D48E, A61T, L136P,S138T, A139F, G179S, E238D, P289L. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE528.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (1): D48E, L136P,G179S, A198G, E238D, P289L. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE530.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (m): D48E, A61T, S138T,A139F, G179S, A198G, P289L. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE531.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (n): D48E, L84P, G111V,S138T, A139T, G179S, A198G, P289L. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE534.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (O): Y28C, D48E, A61T,A89T, S138T, A139T, G179S, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE535.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (p): D48E, A61T, A107T,S108G, L136P, G179S, S192A, E238D, P289L. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE542.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (q): D48E, L136P,G179S, R250W. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE545.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (r): D48E, A89T, S138T,A139T, R163Q, G179S. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE548.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (s): D48E, L136P,G179S, A198G, P289L. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE552.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (t): D48E, F78L, A89T,L136P, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE557.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (u): D48E, A89T, S138T,A139T, G179S, E238D, F278L. Non-limiting examples of plasmids encodingthese amino acid substitutions are pSE564 and pSE565.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (v): D48E, A89T, L136P,R163Q, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE568.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (w): D48E, A61T, A89T,G111V, S138T, A139F, G179S, E238D, P289L. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE543.

In a particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (x): D25G, D48E, A89T,L136P, S138T, A139T, V141A, 1159T, R1630, G179S. A non-limiting exampleof a plasmid encoding these amino acid substitutions is pSE504.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (y): D48E, A89T, S90G,L136P, R163Q, G179S, E238D. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE508.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (z): D48E, A61T, A89T,G111V, S138T, A139T, G179S, E238D, P289L. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE511.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (aa): D48E, A89T,S138T, A139T, G179S. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE520.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ab): D48E, L136P,R163Q, G179S, S231L. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE523.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ac): D48E, L136P,S138T, A139F, G179S, V196A, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE527.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ad): D48E, A61T, A89T,F99S, S138T, A139T, G179S, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE539.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ae): G35S, D48E, A89T,S138T, A139T, G179S, P289L. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE540.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (af): D48E, A61T, A89T,S138T, A139T, G179S, V196A, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE547.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ag): D48E, A89T,G111V, S138T, A139T, G179S, A198G, E238D. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE550.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ah): S41G, D48E, A89T,L136P, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE558.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ai): D48E, A89T,L136P, R163Q, G179S, P252S. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE563.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (aj): D48E, A89T,L136P, G179S, F234S. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE566.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ak): D48E, A89T,L136P, R163Q, G179S, E238D. Non-limiting examples of plasmids encodingthese amino acid substitutions are pSE573 and pSE578.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (al): Q36R, D48E, A89T,L136P, G179S, E238D. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE574.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (am): D48E, A89T,L136P, R1i63Q, G179S. Non-limiting examples of plasmids encoding theseamino acid substitutions are pSE575 and pSE576.

In another particular embodiment, the combination of amino acidsubstitutions comprises the combination of group (an): D48E, A89T,S138T, G179S. A non-limiting example of a plasmid encoding these aminoacid substitutions is pSE577.

In a particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ao): D48E, A89T,L136P, G179S, E238D. Non-limiting examples of plasmids encoding theseamino acid substitutions are pSE502 and pSE524.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ap): D48E, A89T,L136P, K154E, G179S, E238D. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE503.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (aq): D48E, A89T,S138T, A139T, K154R, G179S, 196A, P289L. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE505.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ar): D48E, A89T,S138T, A139F, G179S, V196A, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE506.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (as): D48E, A61T, A89T,L136P, G179S, V196A, A198G, P289L. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE507.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (at): D48E, A61T,S138T, A139F, G179S, G196A, E238D, P289L. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE509.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (au): D48E, A89T,L136P, G179S. Non-limiting examples of plasmids encoding these aminoacid substitutions are pSE514 and pSE525.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (av): D48E, A89T,V120A, L136P, G179S. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE515.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (aw): D48E, A61T, A89T,S138T, A139F, G179S, V196A, A198G, E238D. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE517.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ax): D48E, A61T, A89T,G111V, S138T, A139F, G179S, V196A, E238D. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE518.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ay): D48E, A61T, A89T,S138T, A139T, G179S, V196A, E238D, P289L. Non-limiting examples ofplasmids encoding these amino acid substitutions are pSE529 and pSE554.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (az): D48E, A61T, A89T,L136P, S138T, A139F, G179S, A198G, E238D. A non-limiting example of aplasmid encoding these amino acid substitutions is pSE532.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (ba) D48E, A89T, S138T,A139F, G179S, A198G, V220A. A non-limiting example of a plasmid encodingthese amino acid substitutions is pSE536.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (bb): D48E, A61T, A89T,S138T, A139T, G179S, V196A, E238D, R239H, P289L. A non-limiting exampleof a plasmid encoding these amino acid substitutions is pSE537.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (bc): D48E, A61T, A89T,L136P, G179S, P289L. A non-limiting example of a plasmid encoding theseamino acid substitutions is pSE541.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (bd): D48E, A89T,S138T, A139T, G179S, V196A, E238D, P289L. Non-limiting examples ofplasmids encoding these amino acid substitutions are pSE549 and pSE553.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of group (be): D48E, A61T, A89T,S138T, A139F, G179S, V196A, E238D. A non-limiting example of a plasmidencoding these amino acid substitutions is pSE551.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of groups (da) through (dl):FIG. 6L. A non-limiting example of a plasmid encoding these amino acidsubstitutions is PSE601.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of groups (ea) through (es):FIG. 6M. A non-limiting example of a plasmid encoding these amino acidsubstitutions is pSE617.

The present invention further provides a polynucleotide moleculecomprising a nucleotide sequence that is otherwise the same as theStreptomyces avermitilis aveC allele, the S. avermitilis AveC geneproduct-encoding sequence of plasmid pSE186 (ATCC 209604) or thenucleotide sequence of the aveC ORF of S. avermitilis as presented inFIG. 1 (SEQ ID NO:1), or a degenerate variant thereof, but whichnucleotide sequence further comprises mutations encoding a combinationof amino acid substitutions at amino acid residues corresponding to theamino acid positions of SEQ ID NO:2, such that cells of S. avermitilisstrain ATCC 53692 in which the wild-type aveC allele has beeninactivated and that express a polynucleotide molecule comprising themutated nucleotide sequence are capable of producing a class 2:1 ratioof avermectins that is reduced compared to the ratio produced by cellsof S. avermitilis strain ATCC 53692 that instead express only thewild-type aveC allele, wherein when the class 2:1 avermectins arecyclohexyl B2:cyclohexyl B1 avermectins, the ratio of class 2:1avermectins is reduced to about 0.40:1 or less, and wherein thecombination of amino acid substitutions comprises a combination selectedfrom the group consisting of:

-   -   (bf) D48E, S138T, A139T, G179S, E238D; and    -   (bg) Y28C, Q38R, D48E, L136P, G179S, E238D.

Non-limiting examples of a plasmid encoding the amino acid substitutionsof group (bf) are pSE556 and pSE569. A non-limiting example of a plasmidencoding the amino acid substitutions of group (bg) is pSE561.

In another particular embodiment thereof, the combination of amino acidsubstitutions comprises the combination of groups (ca) through (cs):FIG. 6K. A non-limiting example of a plasmid encoding these amino acidsubstitutions is pSE582.

The present invention contemplates that any of the aforementioned aminoacid substitutions can be accomplished by any modification to thenucleotide sequence of the aveC allele or a degenerate variant thereofthat results in such substitutions. For example, it is possible toeffect most of the amino acid substitutions described herein by changinga native codon sequence or a degenerate variant thereof to any one ofseveral alternative codons that encode the same amino acid substitution.The various possible sequences that can encode the aforementioned aminoacid substitutions will be readily apparent to a person of skill in theart in view of the present disclosure and the known degeneracy of thegenetic code. In a non-limiting embodiment for each particularcombination recited above, the amino acid substitutions are achieved bythe non-silent nucleotide changes set forth in FIG. 6.

As used herein, the phrase “the combination of amino acid substitutionscomprises the combination of group . . . ”, and the like, means that theamino acid substitutions in the AveC gene product according to thepresent invention include at least those substitutions that arespecifically recited, and may include other amino acid substitutions, oramino acid deletions, or amino acid additions, or some combinationthereof, wherein the expression of the resulting AveC gene product inthe S. avermitilis cell yields a desirable reduction in the ratio ofB2:B1 avermectins.

Mutations to the aveC allele or degenerate variant thereof can becarried out by any of a variety of known methods, including by use oferror-prone PCR, or by cassette mutagenesis. For example,oligonucleotide-directed mutagenesis can be employed to alter thesequence of the aveC allele or ORF in a defined way such as, e.g., tointroduce one or more restriction sites, or a termination codon, intospecific regions within the aveC allele or ORF. Methods such as thosedescribed in U.S. Pat. No. 5,605,793, U.S. Pat. No. 5,830,721 and U.S.Pat. No. 5,837,458, which involve random fragmentation, repeated cyclesof mutagenesis, and nucleotide shuffling, can also be used to generatelarge libraries of polynucleotides having nucleotide sequences encodingaveC mutations.

Targeted mutations can be useful, particularly where they serve to alterone or more conserved amino acid residues in the AveC gene product. Forexample, a comparison of the deduced amino acid sequence of the AveCgene product of S. avermitilis (SEQ ID NO:2) with AveC homolog geneproducts from S. griseochromogenes (SEQ ID NO:5) and S. hygroscopicus(SEQ ID NO:4), as described in U.S. Pat. No. 6,248,579, indicates sitesof significant conservation of amino acid residues between thesespecies. Targeted mutagenesis that leads to a change in one or more ofthese conserved amino acid residues may be effective in producing novelmutant strains that exhibit desirable alterations in avermectinproduction.

Random mutagenesis can also be useful, and can be carried out byexposing cells of S. avermitilis to ultraviolet radiation or x-rays, orto chemical mutagens such as N-methyl-N′-nitrosoguanidine, ethyl methanesulfonate, nitrous acid or nitrogen mustards. See, e.g., Ausubel, 1989,above, for a review of mutagenesis techniques.

Once mutated polynucleotide molecules are generated, they are screenedto determine whether they can modulate avermectin biosynthesis in S.avermitilis. In a preferred embodiment, a polynucleotide molecule havinga mutated nucleotide sequence is tested by complementing a strain of S.avermitilis in which the aveC gene has been inactivated to give an aveCnegative (aveC⁻) background. In a non-limiting method, the mutatedpolynucleotide molecule is spliced into an expression plasmid inoperative association with one or more regulatory elements, whichplasmid also preferably comprises one or more drug resistance genes toallow for selection of transformed cells. This vector is thentransformed into aveC⁻host cells using known techniques, and transformedcells are selected and cultured in appropriate fermentation media underconditions that permit or induce avermectin production, for example, byincluding appropriate starter subunits in the medium, and culturingunder optimal conditions for avermectin production as known in the art.Fermentation products are then analyzed by HPLC to determine the abilityof the mutated polynucleotide molecule to complement the host cell.Several plasmid vectors bearing mutated polynucleotide molecules capableof reducing the B2:B1 ratio of avermectins, including pSE188, pSE199,pSE231, pSE239, and pSE290 through pSE297, are exemplified in Section8.3, below. Other examples of such plasmid vectors are recited in FIG.6.

Any of the aforementioned methods of the present invention can becarried out using fermentation culture media preferably supplementedwith cyclohexane carboxylic acid, although other appropriate fatty acidprecursors, such as any one of the fatty acid precursors listed in TABLE1, or methylthiolactic acid, can also used.

Once a mutated polynucleotide molecule that modulates avermectinproduction in a desirable direction has been identified, the location ofthe mutation in the nucleotide sequence can be determined. For example,a polynucleotide molecule having a nucleotide sequence encoding amutated AveC gene product can be isolated by PCR and subjected to DNAsequence analysis using known methods. By comparing the DNA sequence ofthe mutated aveC allele to that of the wild-type aveC allele, themutation(s) responsible for the alteration in avermectin production canbe determined. For example, S. avermitilis AveC gene products comprisingeither single amino acid substitutions at any of residues 55 (S55F), 138(S138T), 139 (A139T), or 230 (G230D), or double substitutions atpositions 138 (S138T) and 139 (A 139T or A139F), yielded changes in AveCgene product function such that the ratio of class 2:1 avermectinsproduced was altered (see Section 8, below), wherein the recited aminoacid positions correspond to those presented in FIG. 1 (SEQ ID NO:2). Inaddition, the following seven combinations of mutations have each beenshown to effectively reduce the class 2:1 ratio of avermectins: (1)D48E/A89T; (2) S138T/A139T/G179S; (3) Q38P/L136P/E238D; (4)F99S/S138T/A139T/G179S; (5) A139T/M228T; (6) G111V/P289L; (7)A139T/K154E/Q298H; (8) D48E/A61 T/R71 L/A89T/L136P/T149S/F176C/G179S/V196A/E238D/I280V. The present invention provides one hundredeight (108) additional combinations of mutations that are shown toreduce the cyclohexyl B2:cyclohexyl B1 ratio of avermectins, and theseare presented in FIG. 6 and recited in the appended claims.

As used herein, the aforementioned designations, such as A139T, indicatethe original amino acid residue by single letter designation, which inthis example is alanine (A), at the indicated position, which in thisexample is position 139 (referring to SEQ ID NO:2) of the polypeptide,followed by the amino acid residue which replaces the original aminoacid residue, which in this example is threonine (T).

As used herein, where an amino acid residue encoded by an aveC allele inthe S. avermitilis chromosome, or in a vector or isolated polynucleotidemolecule of the present invention is referred to as “corresponding to” aparticular amino acid residue of SEQ ID NO:2, or where an amino acidsubstitution is referred to as occurring at a particular position“corresponding to” that of a specific numbered amino acid residue of SEQID NO:2, this is intended to refer to the amino acid residue at the samerelative location in the AveC gene product, which the skilled artisancan quickly determine by reference to the amino acid sequence presentedherein as SEQ ID NO:2.

As used herein, where specific mutations in the aveC allele encodingparticular mutations are recited as base changes at specific nucleotidepositions in the aveC allele “corresponding to” particular nucleotidepositions as shown in SEQ ID NO:1, or where a nucleotide position in theaveC allele is otherwise referred to as “corresponding to” a particularnucleotide position in SEQ ID NO:1, this is intended to refer to thenucleotide at the same relative location in the aveC nucleotide sequenceor a degenerate variant thereof, which the skilled artisan can quicklydetermine by reference to the nucleotide sequence presented herein asSEQ ID NO:1.

As used herein to refer to ratios of cyclohexyl B2:cyclohexyl B1avermectins, the term “about” refers to the specifically statednumerical value plus or minus 10% of that stated value.

A polynucleotide molecule of the present invention may be “isolated”,which means either that it is: (i) purified to the extent that it issubstantially free of other polynucleotide molecules having differentnucleotide sequences, or (ii) present in an environment in which itwould not naturally occur, e.g., where an aveC allele from S.avermitilis, or a mutated version thereof, is present in a cell otherthan a cell of S. avermitilis, or (iii) present in a form in which itwould not naturally occur, e.g., as a shorter piece of DNA, such as arestriction fragment digested out of a bacterial chromosome, comprisingpredominantly the aveC coding region or a mutated version thereof, withor without any associated regulatory sequences thereof, or assubsequently integrated into a heterologous piece of DNA, such as thechromosome of a bacterial cell (other than a cell of S. avermitilis) orthe DNA of a vector such as a plasmid or phage, or integrated into theS. avermitilis chromosome at a locus other than that of the native aveCallele.

The present invention further provides a recombinant vector comprising apolynucleotide molecule of the present invention. Such a recombinantvector can be used to target any of the polynucleotide moleculescomprising mutated nucleotide sequences of the present invention to thesite of the aveC allele of the S. avermitilis chromosome to eitherinsert into or replace the aveC ORF or a portion thereof, e.g., byhomologous recombination. According to the present invention, however, apolynucleotide molecule comprising a mutated nucleotide sequence of thepresent invention provided herewith can also function to modulateavermectin biosynthesis when inserted into the S. avermitilis chromosomeat a site other than at the aveC allele, or when maintained episomallyin S. avermitilis cells. Thus, the present invention further providesvectors comprising a polynucleotide molecule comprising a mutatednucleotide sequence of the present invention, which vectors can be usedto insert the polynucleotide molecule at a site in the S. avermitilischromosome other than at the aveC gene, or to be maintained episomally.

In a non-limiting embodiment, the vector is a gene replacement vectorthat can be used to insert a mutated aveC allele or degenerate variantthereof according to the present invention into cells of a strain of S.avermitilis, thereby generating novel strains of S. avermitilis, thecells of which can produce avermectins in a reduced class 2:1 ratiocompared to cells of the same strain which instead express only thewild-type aveC allele. Such gene replacement vectors can be constructedusing mutated polynucleotide molecules present in expression vectorsprovided herewith, such as those expression vectors exemplified inSection 8 below.

The present invention further provides vectors that can be used toinsert a mutated aveC allele or degenerate variant thereof into cells ofa strain of S. avermitilis to generate novel strains of cells thatproduce altered amounts of avermectins compared to cells of the samestrain which instead express only the wild-type aveC allele. In apreferred embodiment, the amount of avermectins produced by the cells isincreased. In a specific though non-limiting embodiment, such a vectorcomprises a strong promoter as known in the art, such as, e.g., thestrong constitutive ermE promoter from Saccharopolyspora erythraea, thatis situated upstream from, and in operative association with, the aveCORF. Such vectors can be constructed using the mutated aveC allele ofplasmid pSE189, and according to methods described in U.S. Pat. No.6,248,579,

The present invention provides gene replacement vectors that are usefulto inactivate the aveC gene in a wild-type strain of S. avermitilis. Ina non-limiting embodiment, such gene replacement vectors can beconstructed using the mutated polynucleotide molecule present in plasmidpSE180 (ATCC 209605), which is exemplified in Section 8.1, below (FIG.3). The present invention further provides gene replacement vectors thatcomprise a polynucleotide molecule comprising or consisting ofnucleotide sequences that naturally flank the aveC gene in situ in theS. avermitilis chromosome, including, e.g., those flanking nucleotidesequences shown in FIG. 1 (SEQ ID NO:1), which vectors can be used todelete the S. avermitilis aveC ORF.

The present invention further provides a host cell comprising apolynucleotide molecule or recombinant vector of the present invention.The host cell can be any prokaryotic or eukaryotic cell capable of useas a host for the polynucleotide molecule or recombinant vector. In apreferred embodiment, the host cell is a bacterial cell. In a morepreferred embodiment, the host cell is a Streptomyces cell. In a morepreferred embodiment, the host cell is a cell of Streptomycesavermitilis.

The present invention further provides a method for making a novelstrain of Streptomyces avermitilis, comprising (i) mutating the aveCallele in a cell of a strain of S. avermitilis, which mutation resultsin a combination of amino acid substitutions in the AveC gene product,or (ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein thecombination of amino acid substitutions is selected from (a) through(es) listed above.

The present invention further provides a method for making a novelstrain of S. avermitilis, comprising (i) mutating the aveC allele in acell of a strain of S. avermitilis, which mutation results in acombination of amino acid substitutions in the AveC gene product, or(ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein cellscomprising the mutated aveC allele or degenerate variant are capable ofproducing cyclohexyl B2:cyclohexyl B1 avermectins in a ratio of 0.35:1or less. In a non-limiting embodiment thereof, the mutated aveC alleleor degenerate variant thereof encodes an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (a) through (be) and (da) through (es) listed above.

In a preferred embodiment thereof, the ratio of cyclohexyl B2:cyclohexylB1 avermectins is about 0.30:1 or less. In a non-limiting embodimentthereof, the mutated aveC allele or degenerate variant thereof encodesan AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (f) through (be) and(da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.25:1 or less. In a non-limitingembodiment thereof, the mutated aveC allele or degenerate variantthereof encodes an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (w) through(be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.20:1 or less. In a non-limitingembodiment thereof, the mutated aveC allele or degenerate variantthereof encodes an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (ao) through(be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.10:1 or less. In a non-limitingembodiment thereof, the mutated aveC allele or degenerate variantthereof encodes an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (ea) through(es) listed above.

The present invention further provides a method for making a novelstrain of Streptomyces avermitilis, comprising (i) mutating the aveCallele in a cell of a strain of S. avermitilis, which mutation resultsin a combination of amino acid substitutions in the AveC gene product,or (ii) introducing into a cell of a strain of S. avermitilis a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions, wherein thecombination of amino acid substitutions is selected from the groupconsisting of (bf) and (bg). In a preferred embodiment thereof, cells ofS. avermitilis comprising such a mutated aveC allele or degeneratevariant are capable of producing cyclohexyl B2:cyclohexyl B1 avermectinsin a ratio of about 0.40:1 or less.

By so mutating the aveC allele, or by so introducing a mutated aveCallele or degenerate variant thereof, according to the above-recitedsteps, a new strain of S. avermitilis is made.

The present invention further provides a cell of a Streptomyces speciesthat comprises a mutated aveC allele or degenerate variant thereofencoding an AveC gene product comprising a combination of amino acidsubstitutions selected from (a) through (es) listed above. In apreferred embodiment thereof, the species of Streptomyces is S.avermitilis.

The present invention further provides a cell of S. avermitilis capableof producing cyclohexyl B2:cyclohexyl B1 avermectins in a ratio of0.35:1 or less. In a non-limiting embodiment thereof, the cell comprisesa mutated aveC allele or degenerate variant thereof encoding an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (a) through (be) and (da) through(es) listed above.

In a preferred embodiment thereof, the ratio of cyclohexyl B2:cyclohexylB1 avermectins is about 0.30:1 or less. In a non-limiting embodimentthereof, the cell comprises a mutated aveC allele or degenerate variantthereof encoding an AveC gene product comprising a combination of aminoacid substitutions selected from the group consisting of (f) through(be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.25:1 or less. In a non-limitingembodiment thereof, the cell comprises a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (w) through (be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.20:1 or less. In a non-limitingembodiment thereof, the cell comprises a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (ao) through (be) and (da) through (es) listed above.

In a more preferred embodiment thereof, the ratio of cyclohexylB2:cyclohexyl B1 avermectins is about 0.10:1 or less. In a non-limitingembodiment thereof, the cell comprises a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (ea) through (es) listed above.

The present invention further provides a cell of a Streptomyces species,comprising a mutated aveC allele or degenerate variant thereof encodingan AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (bf) and (bg) listedabove. In a preferred embodiment thereof, the species of Streptomyces isS. avermitilis. In a more preferred embodiment thereof, the cell is acell of S. avermitilis capable of producing cyclohexyl B2:cyclohexyl B1avermectins in a ratio of about 0.40:1 or less.

Although any of the recited mutations can be present in cells of thepresent invention on an extrachromosomal element such as a plasmid, itis preferred that such mutations are present in an aveC coding sequenceintegrated into the S. avermitilis chromosome, and preferably, thoughnot necessarily, at the site of the native aveC allele.

Such novel strains of cells are useful in the large-scale production ofcommercially desirable avermectins such as doramectin.

The present invention further provides a process for producingavermectins, comprising culturing the S. avermitilis cells of thepresent invention in culture media under conditions that permit orinduce the production of avermectins therefrom, and recovering saidavermectins from the culture. In a preferred embodiment, the cells usedin the process produce cyclohexyl B2:cyclohexyl B1 avermectins in aratio of 0.35:1 or less, more preferably in a ratio of about 0.30:1 orless, more preferably in a ratio of about 0.25:1 or less, morepreferably in a ratio of about 0.20:1 or less, and more preferably in aratio of about 0.10 or less.

In a preferred embodiment thereof, cells producing cyclohexylB2:cyclohexyl B1 avermectins in a ratio of 0.35:1 or less comprise amutated aveC allele or degenerate variant thereof encoding an AveC geneproduct comprising a combination of amino acid substitutions selectedfrom the group consisting of (a) through (be) and (da) through (es)listed above.

In a further preferred embodiment thereof, cells producing cyclohexylB2:cyclohexyl B1 avermectins in a ratio of about 0.30:1 or less comprisea mutated aveC allele or degenerate variant thereof encoding an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (f) through (be) and (da) through(es) listed above.

In a further preferred embodiment thereof, cells producing cyclohexylB2:cyclohexyl B1 avermectins in a ratio of about 0.25:1 or less comprisea mutated aveC allele or degenerate variant thereof encoding an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (w) through (be) and (da) through(es) listed above.

In a further preferred embodiment thereof, cells producing cyclohexylB2:cyclohexyl B1 avermectins in a ratio of about 0.20:1 or less comprisea mutated aveC allele or degenerate variant thereof encoding an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (ao) through (be) and (da) through(es) listed above.

In a further preferred embodiment thereof, cells producing cyclohexylB2:cyclohexyl B1 avermectins in a ratio of about 0.10:1 or less comprisea mutated aveC allele or degenerate variant thereof encoding an AveCgene product comprising a combination of amino acid substitutionsselected from the group consisting of (ea) through (es) listed above.

In another embodiment, the cells produce cyclohexyl B2:cyclohexyl B1avermectins in a ratio of about 0.40:1 or less and comprise a mutatedaveC allele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (bf) and (bg) listed above.

The process of the invention provides increased efficiency in theproduction of commercially valuable avermectins such as doramectin.

The present invention further provides a composition of cyclohexylB2:cyclohexyl B1 avermectins produced by cells of Streptomycesavermitilis, comprising the cyclohexyl B2:cyclohexyl B1 avermectinspresent in a culture medium in which the cells have been cultured,wherein the ratio of the cyclohexyl B2:cyclohexyl B1 avermectins presentin the culture medium is 0.35:1 or less, preferably about 0.30:1 orless, more preferably about 0.25:1 or less, more preferably about 0.20:1or less, and more preferably about 0.10 and less. In a particularembodiment, the composition of cyclohexyl B2:cyclohexyl B1 avermectinsis produced by cells of a strain of S. avermitilis that express amutated aveC allele or degenerate variant thereof which encodes a geneproduct that results in the reduction in the ratio of cyclohexylB2:cyclohexyl B1 avermectins produced by the cells compared to cells ofthe same strain of S. avermitilis that do not express the mutated aveCallele but instead express only the wild-type aveC allele.

In a preferred embodiment thereof, where the composition is cyclohexylB2:cyclohexyl B1 avermectins in a ratio of 0.35:1 or less, thecomposition is produced by cells comprising a mutated aveC allele ordegenerate variant thereof encoding an AveC gene product comprising acombination of amino acid substitutions selected from the groupconsisting of (a) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.30:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (f) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.25:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (w) through (be) and (da) through (es) listed above.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.20:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (ao) through (be) and (da) through (es) listedabove.

In a further preferred embodiment thereof, where the composition iscyclohexyl B2:cyclohexyl B1 avermectins in a ratio of about 0.10:1 orless, the composition is produced by cells comprising a mutated aveCallele or degenerate variant thereof encoding an AveC gene productcomprising a combination of amino acid substitutions selected from thegroup consisting of (ea) through (es) listed above.

The present invention further provides a composition of cyclohexylB2:cyclohexyl B1 avermectins produced by cells of Streptomycesavermitilis, comprising the cyclohexyl B2:cyclohexyl B1 avermectinspresent in a culture medium in which the cells have been cultured,wherein the ratio of the cyclohexyl B2:cyclohexyl B1 avermectins presentin the culture medium is about 0.40:1 or less, and which is produced bycells comprising a mutated aveC allele or degenerate variant thereofencoding an AveC gene product comprising a combination of amino acidsubstitutions selected from the group consisting of (bf) and (bg) listedabove.

Although it is preferred that the novel avermectin composition ispresent in a culture medium in which the cells have been cultured, e.g.,in partially or totally exhausted fermentation culture fluid, theavermectin composition may alternatively be partially or substantiallypurified from the culture fluid by known biochemical techniques ofpurification, such as by ammonium sulfate precipitation, dialysis, sizefractionation, ion exchange chromatography, HPLC, etc.

In addition to making novel strains of S. avermitilis comprising cellsthat are capable of producing reduced ratios of cyclohexyl B2:cyclohexylB1 as described above, the present invention contemplates thatadditional mutations can be incorporated into cells of S. avermitilis tofurther improve characteristics of avermectin production. In anon-limiting embodiment, cells of the present invention can furthercomprise modifications to increase the production level of avermectins.In one embodiment, such cells can be prepared by (i) mutating the aveCallele in a cell of S. avermitilis, or (ii) introducing a mutated aveCallele or degenerate variant thereof into cells of a strain of S.avermitilis, wherein the expression of the mutated allele results in anincrease in the amount of avermectins produced by cells of a strain ofS. avermitilis expressing the mutated aveC allele compared to cells ofthe same strain that instead express only a single wild-type aveCallele, and selecting transformed cells that produce avermectins in anincreased amount compared to the amount of avermectins produced by cellsof the strain that instead express only the single wild-type aveCallele. For example, the aveC allele can be modified so that itcomprises a strong promoter, such as the strong constitutive ermEpromoter from Saccharopolyspora erythraea, inserted upstream from and inoperative association with the aveC ORF. In another embodiment, one ormore mutations can be introduced into the aveR1 and/or aveR2 genes of S.avermitilis, thereby increasing the level of avermectin production asdescribed in U.S. Pat. No. 6,197,591 to Stutzman-Engwall et al., issuedMar. 6, 2001.

5.4. Uses of Avermectins

Avermectins are highly active antiparasitic agents having particularutility as anthelmintics, ectoparasiticides, insecticides andacaricides. Avermectin compounds produced according to the methods ofthe present invention are useful for any of these purposes. For example,avermectin compounds produced according to the present invention areuseful to treat various diseases or conditions in humans, particularlywhere those diseases or conditions are caused by parasitic infections,as known in the art. See, e.g., Ikeda and Omura, 1997, Chem. Rev.97(7):2591-2609. More particularly, avermectin compounds producedaccording to the present invention are effective in treating a varietyof diseases or conditions caused by endoparasites, such as parasiticnematodes, which can infect humans, domestic animals, swine, sheep,poultry, horses or cattle.

More specifically, avermectin compounds produced according to thepresent invention are effective against nematodes that infect humans, aswell as those that infect various species of animals. Such nematodesinclude gastrointestinal parasites such as Ancylostoma, Necator,Ascaris, Strongyloides, Trichinella, Capillaria, Trichuris, Enterobius,Dirofilaria, and parasites that are found in the blood or other tissuesor organs, such as filarial worms and the extract intestinal states ofStrongyloides and Trichinella.

The avermectin compounds produced according to the present invention arealso useful in treating ectoparasitic infections including, e.g.,arthropod infestations of mammals and birds, caused by ticks, mites,lice, fleas, blowflies, biting insects, or migrating dipterous larvaethat can affect cattle and horses, among others.

The avermectin compounds produced according to the present invention arealso useful as insecticides against household pests such as, e.g., thecockroach, clothes moth, carpet beetle and the housefly among others, aswell as insect pests of stored grain and of agricultural plants, whichpests include spider mites, aphids, caterpillars, and orthopterans suchas locusts, among others.

Animals that can be treated with the avermectin compounds producedaccording to the present invention include sheep, cattle, horses, deer,goats, swine, birds including poultry, and dogs and cats.

An avermectin compound produced according to the present invention isadministered in a formulation appropriate to the specific intended use,the particular species of host animal being treated, and the parasite orinsect involved. For use as a parasiticide, an avermectin compoundproduced according to the present invention can be administered orallyin the form of a capsule, bolus, tablet or liquid drench or,alternatively, can be administered as a pour-on, or by injection, or asan implant. Such formulations are prepared in a conventional manner inaccordance with standard veterinary practice. Thus, capsules, boluses ortablets can be prepared by mixing the active ingredient with a suitablefinely divided diluent or carrier additionally containing adisintegrating agent and/or binder such as starch, lactose, talc,magnesium stearate, etc. A drench formulation can be prepared bydispersing the active ingredient in an aqueous solution together with adispersing or wetting agent, etc. Injectable formulations can beprepared in the form of a sterile solution, which can contain othersubstances such as, e.g., sufficient salts and/or glucose to make thesolution isotonic with blood.

Such formulations will vary with regard to the weight of active compounddepending on the patient, or species of host animal to be treated, theseverity and type of infection, and the body weight of the host.Generally, for oral administration a dose of active compound of fromabout 0.001 to 10 mg per kg of patient or animal body weight given as asingle dose or in divided doses for a period of from 1 to 5 days will besatisfactory. However, there can be instances where higher or lowerdosage ranges are indicated, as determined, e.g., by a physician orveterinarian, as based on clinical symptoms.

As an alternative, an avermectin compound produced according to thepresent invention can be administered in combination with animalfeedstuff, and for this purpose a concentrated feed additive or premixcan be prepared for mixing with the normal animal feed.

For use as an insecticide, and for treating agricultural pests, anavermectin compound produced according to the present invention can beapplied as a spray, dust, emulsion and the like in accordance withstandard agricultural practice.

6. EXAMPLE Fermentation of Streptomyces avermitilis and B2:B1 AvermectinAnalysis

Strains lacking both branched-chain 2-oxo acid dehydrogenase and5-O-methyltransferase activities produce no avermectins if thefermentation medium is not supplemented with fatty acids. This exampledemonstrates that in such mutants a wide range of B2:B1 ratios ofavermectins can be obtained when biosynthesis is initiated in thepresence of different fatty acids.

6.1. Materials and Methods

Streptomyces avermitilis ATCC 53692 was stored at −70° C. as a wholebroth prepared in seed medium consisting of: Starch (Nadex, LaingNational)—20g; Pharmamedia (Trader's Protein, Memphis, Tenn.)—15 g;Ardamine pH (Yeast Products Inc.)—5 g; calcium carbonate—1 g. Finalvolume was adjusted to 1 liter with tap water, pH was adjusted to 7.2,and the medium was autoclaved at 121° C. for 25 min.

Two ml of a thawed suspension of the above preparation was used toinoculate a flask containing 50 ml of the same medium. After 48 hrsincubation at 28° C. on a rotary shaker at 180 rpm, 2 ml of the brothwas used to inoculate a flask containing 50 ml of a production mediumconsisting of: Starch—80 g; calcium carbonate—7 g; Pharmamedia—5 g;dipotassium hydrogen phosphate—1 g; magnesium sulfate—1 g; glutamicacid—0.69; ferrous sulfate heptahydrate—0.01 g; zinc sulfate—0.0019;manganous sulfate—0.001 g. Final volume was adjusted to 1 liter with tapwater, pH was adjusted to 7.2, and the medium was autoclaved at 121° C.for 25 min.

Various carboxylic acid substrates (see TABLE 1) were dissolved inmethanol and added to the fermentation broth 24 hrs after inoculation togive a final concentration of 0.2 g/liter. The fermentation broth wasincubated for 14 days at 28° C., then the broth was centrifuged (2,500rpm for 2 min) and the supernatant discarded. The mycelial pellet wasextracted with acetone (15 ml), then with dichloromethane (30 ml), andthe organic phase separated, filtered, then evaporated to dryness. Theresidue was taken up in methanol (1 ml) and analyzed by HPLC with aHewlett-Packard 1090A liquid chromatograph equipped with a scanningdiode-array detector set at 240 nm. The column used was a BeckmanUltrasphere C-18, 5 μm, 4.6 mm×25 cm column maintained at 40° C.Twenty-five μl of the above methanol solution was injected onto thecolumn. Elution was performed with a linear gradient of methanol-waterfrom 80:20 to 95:5 over 40 min at 0.85/ml min. Two standardconcentrations of cyclohexyl B1 were used to calibrate the detectorresponse, and the area under the curves for B2 and B1 avermectins wasmeasured.

6.2. Results

The HPLC retention times observed for the B2 and B1 avermectins, and the2:1 ratios, are shown in TABLE 1.

TABLE 1 HPLC Retention Time (min) Ratio Substrate B2 B1 B2:B14-Tetrahydropyran carboxylic acid 8.1 14.5 0.25 Isobutyric acid 10.818.9 0.5 3-Furoic acid 7.6 14.6 0.62 S-(+)-2-methylbutyric acid 12.821.6 1.0 Cyclohexanecarboxylic acid 16.9 26.0 1.6 3-Thiophenecarboxylicacid 8.8 16.0 1.8 Cyclopentanecarboxylic acid 14.2 23.0 2.03-Trifluoromethylbutyric acid 10.9 18.8 3.9 2-Methylpentanoic acid 14.524.9 4.2 Cycloheptanecarboxylic acid 18.6 29.0 15.0

The data presented in TABLE 1 demonstrates an extremely wide range ofB2:B1 avermectin product ratios, indicating a considerable difference inthe results of dehydrative conversion of class 2 compounds to class 1compounds, depending on the nature of the fatty acid side chain starterunit supplied. This indicates that changes in B2:B1 ratios resultingfrom alterations to the AveC protein may be specific to particularsubstrates. Consequently, screening for mutants exhibiting changes inthe B2:B1 ratio obtained with a particular substrate needs to be done inthe presence of that substrate. The subsequent examples described belowuse cyclohexanecarboxylic acid as the screening substrate. However, thissubstrate is used merely to exemplify the potential, and is not intendedto limit the applicability, of the present invention.

7. EXAMPLE Isolation of the aveC Gene

This example describes the isolation and characterization of a region ofthe Streptomyces avermitilis chromosome that encodes the AveC geneproduct. As demonstrated below, the aveC gene was identified as capableof modifying the ratio of cyclohexyl-B2 to cyclohexyl-B1 (B2:B1)avermectins produced.

7.1. Materials and Methods 7.1.1. Growth of Streptomyces for DNAIsolation

The following method was followed for growing Streptomyces. Singlecolonies of S. avermitilis ATCC 31272 (single colony isolate #2) wereisolated on 1/2 strength YPD-6 containing: Difco Yeast Extract—5 g;Difco Bacto-peptone—5 g; dextrose—2.5 g; MOPS—5 g; Difco Bacto agar—15g. Final volume was adjusted to 1 liter with dH₂O, pH was adjusted to7.0, and the medium was autoclaved at 121° C. for 25 min.

The mycelia grown in the above medium were used to inoculate 10 ml ofTSB medium (Difco Tryptic Soy Broth—30 g, in 1 liter dH₂O, autoclaved at121° C. for 25 min) in a 25 mm×150 mm tube which was maintained withshaking (300 rpm) at 28° C. for 48-72 hrs.

7.1.2. Chromosomal DNA Isolation from Streptomyces

Aliquots (0.25 ml or 0.5 ml) of mycelia grown as described above wereplaced in 1.5 ml microcentrifuge tubes and the cells concentrated bycentrifugation at 12,000×g for 60 sec. The supernatant was discarded andthe cells were resuspended in 0.25 ml TSE buffer (20 ml 1.5 M sucrose,2.5 ml 1 M Tris-HCl, pH 8.0, 2.5 ml 1 M EDTA, pH 8.0, and 75 ml dH₂O)containing 2 mg/ml lysozyme. The samples were incubated at 37° C. for 20min with shaking, loaded into an AutoGen 540™ automated nucleic acidisolation instrument (Integrated Separation Systems, Natick, Mass.), andgenomic DNA isolated using Cycle 159 (equipment software) according tomanufacturer's instructions.

Alternatively, 5 ml of mycelia were placed in a 17 mm×100 mm tube, thecells concentrated by centrifugation at 3,000 rpm for 5 min, and thesupernatant removed. Cells were resuspended in 1 ml TSE buffer,concentrated by centrifugation at 3,000 rpm for 5 min, and thesupernatant removed. Cells were resuspended in 1 ml TSE buffercontaining 2 mg/ml lysozyme, and incubated at 37° C. with shaking for30-60 min. After incubation, 0.5 ml 10% sodium dodecyl sulfate (SDS) wasadded and the cells incubated at 37° C. until lysis was complete. Thelysate was incubated at 65° C. for 10 min, cooled to rm temp, split intotwo 1.5 ml Eppendorf tubes, and extracted 1×with 0.5 mlphenol/chloroform (50% phenol previously equilibrated with 0.5 M Tris,pH 8.0; 50% chloroform). The aqueous phase was removed and extracted 2to 5×with chloroform:isoamyl alcohol (24:1). The DNA was precipitated byadding 1/10 volume 3M sodium acetate, pH 4.8, incubating the mixture onice for 10 min, centrifuging the mixture at 15,000 rpm at 5° C. for 10min, and removing the supernatant to a clean tube to which 1 volume ofisopropanol was added. The supernatant plus isopropanol mixture was thenincubated on ice for 20 min, centrifuged at 15,000 rpm for 20 min at 5°C., the supernatant removed, and the DNA pellet washed 1×with 70%ethanol. After the pellet was dry, the DNA was resuspended in TE buffer(10 mM Tris, 1 mM EDTA, pH 8.0).

7.1.3. Plasmid DNA Isolation from Streptomyces

An aliquot (1.0 ml) of mycelia was placed in 1.5 ml microcentrifugetubes and the cells concentrated by centrifugation at 12,000×g for 60sec. The supernatant was discarded, the cells were resuspended in 1.0 ml10.3% sucrose and concentrated by centrifugation at 12,000×g for 60 sec,and the supernatant discarded. The cells were then resuspended in 0.25ml TSE buffer containing 2 mg/ml lysozyme, and incubated at 37° C. for20 min with shaking and loaded into the AutoGen 540TM automated nucleicacid isolation instrument. Plasmid DNA was isolated using Cycle 106(equipment software) according to manufacturer's instructions.

Alternatively, 1.5 ml of mycelia were placed in 1.5 ml microcentrifugetubes and the cells concentrated by centrifugation at 12,000×g for 60sec. The supernatant was discarded, the cells were resuspended in 1.0 ml10.3% sucrose and concentrated by centrifugation at 12,000×g for 60 sec,and the supernatant discarded. The cells were resuspended in 0.5 ml TSEbuffer containing 2 mg/ml lysozyme, and incubated at 37° C. for 15-30min. After incubation, 0.25 ml alkaline SDS (0.3N NaOH, 2% SDS) wasadded and the cells incubated at 55° C. for 15-30 min or until thesolution was clear. Sodium acetate (0.1 ml, 3M, pH 4.8) was added to theDNA solution, which was then incubated on ice for 10 min. The DNAsamples were centrifuged at 14,000 rpm for 10 min at 5° C. Thesupernatant was removed to a clean tube, and 0.2 ml phenol:chloroform(50% phenol:50% chloroform) was added and gently mixed. The DNA solutionwas centrifuged at 14,000 rpm for 10 min at 5° C. and the upper layerremoved to a clean Eppendorf tube. Isopropanol (0.75 ml) was added, andthe solution was gently mixed and then incubated at rm temp for 20 min.The DNA solution was centrifuged at 14,000 rpm for 15 min at 5° C., thesupernatant removed, and the DNA pellet was washed with 70% ethanol,dried, and resuspended in TE buffer.

7.1.4. Plasmid DNA Isolation from E. coli

A single transformed E. coli colony was inoculated into 5 mlLuria-Bertani (LB) medium (Bacto-Tryptone—10 g, Bacto-yeast extract—5 g,and aCl—10 g in 1 liter dH₂O, pH 7.0, autoclaved at 121° C. for 25 min,and supplemented with 100 μg/ml ampicillin). The culture was incubatedovernight, and a 1 ml aliquot placed in a 1.5 ml microcentrifuge tube.The culture samples were loaded into the AutoGen 540TM automated nucleicacid isolation instrument and plasmid DNA was isolated using Cycle 3(equipment software) according to manufacturer's instructions.

7.1.5. Preparation and Transformation of S. avermitilis Protoplasts

Single colonies of S. avermitilis were isolated on ½ strength YPD-6. Themycelia were used to inoculate 10 ml of TSB medium in a 25 mm×150 mmtube, which was then incubated with shaking (300 rpm) at 28° C. for 48hrs. One ml of mycelia was used to inoculate 50 ml YEME medium. YEMEmedium contains per liter: Difco Yeast Extract—3 g; DifcoBacto-peptone—5 g; Difco Malt Extract—3 g; Sucrose—300 g. Afterautoclaving at 121° C. for 25 min, the following were added: 2.5 MMgCl₂, 6H₂O (separately autoclaved at 121° C. for 25 min)—2 ml; andglycine (20%) (filter-sterilized)-25 ml.

The mycelia were grown at 30° C. for 48-72 hrs and harvested bycentrifugation in a 50 ml centrifuge tube (Falcon) at 3,000 rpm for 20min. The supernatant was discarded and the mycelia were resuspended in Pbuffer, which contains: sucrose—205 g; K₂SO₄—0.25 g; MgCl₂·6H₂O—2.02 g;H₂O—600 ml; K₂PO₄ (0.5%)—10 ml; trace element solution—20 ml; CaCl₂·2H₂O(3.68%)—100 ml; and MES buffer (1.0 M, pH 6.5)—10 ml. (*Trace elementsolution contains per liter: ZnCl₂—40 mg; FeCl₃.6H₂O—200 mg; CuCl₂.2H₂O—10 mg; MnCl₂·4H₂O—10 mg; Na₂B₄O₇·10H₂O—10 mg; (NH₄)₆ Mo₇O₂₄·4H₂O—10mg). The pH was adjusted to 6.5, final volume was adjusted to 1 liter,and the medium was filtered hot through a 0.45 micron filter.

The mycelia were pelleted at 3,000 rpm for 20 min, the supernatant wasdiscarded, and the mycelia were resuspended in 20 ml P buffer containing2 mg/ml lysozyme. The mycelia were incubated at 35° C. for 15 min withshaking, and checked microscopically to determine extent of protoplastformation. When protoplast formation was complete, the protoplasts werecentrifuged at 8,000 rpm for 10 min. The supernatant was removed and theprotoplasts were resuspended in 10 ml P buffer. The protoplasts werecentrifuged at 8,000 rpm for 10 min, the supernatant was removed, theprotoplasts were resuspended in 2 ml P buffer, and approximately 1×10⁹protoplasts were distributed to 2.0 ml cryogenic vials (Nalgene).

A vial containing 1×10⁹ protoplasts was centrifuged at 8,000 rpm for 10min, the supernatant was removed, and the protoplasts were resuspendedin 0.1 ml P buffer. Two to 5 μg of transforming DNA were added to theprotoplasts, immediately followed by the addition of 0.5 ml working Tbuffer. T buffer base contains: PEG-1000 (Sigma)—25 g; sucrose—2.5 g;H₂O—83 ml. The pH was adjusted to 8.8 with 1 N NaOH (filter sterilized),and the T buffer base was filter-sterilized and stored at 4° C. WorkingT buffer, made the same day used, was T buffer base—8.3 ml; K₂PO₄ (4mM)—1.0 ml; CaCl₂·2H₂O (5 M)—0.2 ml; and TES (1 M, pH 8)—0.5 ml. Eachcomponent of the working T buffer was individually filter-sterilized.

Within 20 sec of adding T buffer to the protoplasts, 1.0 ml P buffer wasalso added and the protoplasts were centrifuged at 8,000 rpm for 10 min.The supernatant was discarded and the protoplasts were resuspended in0.1 ml P buffer. The protoplasts were then plated on RM14 media, whichcontains: sucrose—205 g; K₂SO₄.0.25 g; MgCl₂.6H₂O—10.12 g; glucose—10 g;Difco Casamino Acids—0.1 g; Difco Yeast Extract—5 g; Difco OatmealAgar—3 g; Difco Bacto Agar—22 g; dH₂O—800 ml. The solution wasautoclaved at 121° C. for 25 min. After autoclaving, sterile stocks ofthe following were added: K₂PO₄ (0.5%)—10 ml; CaCl₂·2H₂O (5 M)—5 ml;L-proline (20%)—15 ml; MES buffer (1.0 M, pH 6.5)—10 ml; trace elementsolution (same as above)—2 ml; cycloheximide stock (25 mg/ml)—40 ml; and1N NaOH—2 ml. Twenty-five ml of RM14 medium were aliquoted per plate,and plates dried for 24 hr before use.

The protoplasts were incubated in 95% humidity at 30° C. for 20-24 hrs.To select thiostrepton resistant transformants, 1 ml of overlay buffercontaining 125 μg per ml thiostrepton was spread evenly over the RM14regeneration plates. Overlay buffer contains per 100 ml: sucrose—10.3 g;trace element solution (same as above)—0.2 ml; and MES (1 M, pH 6.5)—1ml. The protoplasts were incubated in 95% humidity at 30° C. for 7-14days until thiostrepton resistant (Thio^(r)) colonies were visible.

7.1.6. Transformation of Streptomyces lividans Protoplasts

S. lividans TK64 (provided by the John Innes Institute, Norwich, U.K)was used for transformations in some cases. Methods and compositions forgrowing, protoplasting, and transforming S. lividans are described inHopwood et al., 1985, Genetic Manipulation of Streptomyces, A LaboratoryManual, John Innes Foundation, Norwich, U.K., and performed as describedtherein. Plasmid DNA was isolated from S. lividans transformants asdescribed in Section 7.1.3, above.

7.1.7. Fermentation Analysis of S. avermitilis Strains

S. avermitilis mycelia grown on ½ strength YPD-6 for 4-7 days wereinoculated into 1×6 inch tubes containing 8 ml of preform medium and two5 mm glass beads. Preform medium contains: soluble starch (either thinboiled starch or KOSO, Japan Corn Starch Co., Nagoya)—20 g/L;Pharmamedia—15 g/L; Ardamine pH—5 g/L (Champlain Ind., Clifton, N.J.);CaCO₃—2 g/L; 2×bcfa (“bcfa” refers to ranched chain fatty acids)containing a final concentration in the medium of 50 ppm 2—(+/−)-methylbutyric acid, 60 ppm isobutyric acid, and 20 ppm isovaleric acid. The pHwas adjusted to 7.2, and the medium was autoclaved at 121° C. for 25min.

The tube was shaken at a 170 angle at 215 rpm at 29° C. for 3 days. A 2ml aliquot of the seed culture was used to inoculate a 300 ml Erlenmeyerflask containing 25 ml of production medium which contains: starch(either thin boiled starch or KOSO)—160 g/L; Nutrisoy (Archer DanielsMidland, Decatur, Ill.)—10 g/L; Ardamine pH—10 g/L; K₂HPO₄—2 g/L;MgSO₄.4H₂O—2 g/L; FeSO₄.7H₂O—0.02 g/L; MnCl₂—0.002 g/L; ZnSO₄.7H₂O—0.002g/L; CaCO₃—14 g/L; 2×bcfa (as above); and cyclohexane carboxylic acid(CHC) (made up as a 20% solution at pH 7.0)—800 ppm. The pH was adjustedto 6.9, and the medium was autoclaved at 121° C. for 25 min. (Asexplained above, starter units other than CHC can be utilized instead(see, e.g., Table 1)).

After inoculation, the flask was incubated at 29° C. for 12 days withshaking at 200 rpm. After incubation, a 2 ml sample was withdrawn fromthe flask, diluted with 8 ml of methanol, mixed, and the mixturecentrifuged at 1,250×g for 10 min to pellet debris. The supernatant wasthen assayed by HPLC using a Beckman Ultrasphere ODS column (25 cm×4.6mm ID) with a flow rate of 0.75 ml/min and detection by absorbance at240 nm. The mobile phase was 86/8.9/5.1 methanol/water/acetonitrile.

7.1.8. Isolation of S. avermitilis PKS Genes

A cosmid library of S. avermitilis (ATCC 31272, SC-2) chromosomal DNAwas prepared and hybridized with a ketosynthase (KS) probe made from afragment of the Saccharopolyspora erythraea polyketide synthase (PKS)gene. A detailed description of the preparation of cosmid libraries canbe found in Sambrook et al., 1989, above. A detailed description of thepreparation of Streptomyces chromosomal DNA libraries is presented inHopwood et al., 1985, above. Cosmid clones containingketosynthase-hybridizing regions were identified by hybridization to a2.7 Kb NdeI/Eco47III fragment from pEX26 (kindly supplied by Dr. P.Leadlay, Cambridge, UK). Approximately 5 ng of pEX26 were digested usingNdeI and Eco47III. The reaction mixture was loaded on a 0.8% SeaPlaqueGTG agarose gel (FMC BioProducts, Rockland, Me.). The 2.7 KbNdeI/Eco47III fragment was excised from the gel after electrophoresisand the DNA recovered from the gel using GELase™ from EpicentreTechnologies using the Fast Protocol. The 2.7 Kb NdeI/Eco47III fragmentwas labeled with [α-³²P]dCTP (deoxycytidine 5′-triphosphate, tetra(triethylammonium) salt, [alpha-³²P]-) (NEN-Dupont, Boston, Mass.) usingthe BRL Nick Translation System (BRL Life Technologies, Inc.,Gaithersburg, Md.) following the supplier's instructions. A typicalreaction was performed in 0.05 ml volume. After addition of 5 μl Stopbuffer, the labeled DNA was separated from unincorporated nucleotidesusing a G-25 Sephadex Quick Spin™ Column (Boehringer Mannheim) followingsupplier's instructions.

Approximately 1,800 cosmid clones were screened by colony hybridization.Ten clones were identified that hybridized strongly to the Sacc.erythraea KS probe. E. coli colonies containing cosmid DNA were grown inLB liquid medium and cosmid DNA was isolated from each culture in theAutoGen 540™ automated nucleic acid isolation instrument using Cycle 3(equipment software) according to manufacturer's instructions.Restriction endonuclease mapping and Southern blot hybridizationanalyses revealed that five of the clones contained overlappingchromosomal regions. An S. avermitilis genomic BamHI restriction map ofthe five cosmids (i.e., pSE65, pSE66, pSE67, pSE68, pSE69) wasconstructed by analysis of overlapping cosmids and hybridizations (FIG.4).

7.1.9. Identification of DNA That Modulates Avermectin B2:B1 Ratios andIdentification of an aveC ORF

The following methods were used to test subcloned fragments derived fromthe pSE66 cosmid clone for their ability to modulate avermectin B2:B1ratios in AveC mutants. pSE66 (5 μg) was digested with SacI and BamHI.The reaction mixture was loaded on a 0.8% SeaPlaque™ GTG agarose gel(FMC BioProducts), a 2.9 Kb SacI/BamHI fragment was excised from the gelafter electrophoresis, and the DNA was recovered from the gel usingGELase™ (Epicentre Technologies) using the Fast Protocol. Approximately5 μg of the shuttle vector pWHM3 (Vara et al., 1989, J. Bacteriol.171:5872-5881) was digested with SacI and BamHI. About 0.5 μg of the 2.9Kb insert and 0.5 μg of digested pWHM3 were mixed together and incubatedovernight with 1 unit of ligase (New England Biolabs, Inc., Beverly,Mass.) at 15° C., in a total volume of 20 μl, according to supplier'sinstructions. After incubation, 5 μl of the ligation mixture wasincubated at 70° C. for 10 min, cooled to rm temp, and used to transformcompetent E. coli DH5α cells (BRL) according to manufacturer'sinstructions. Plasmid DNA was isolated from ampicillin resistanttransformants and the presence of the 2.9 Kb SacI/BamHI insert wasconfirmed by restriction analysis. This plasmid was designated aspSE119.

Protoplasts of S. avermitilis strain 1100-SC38 (Pfizer in-house strain)were prepared and transformed with pSE119 as described in Section 7.1.5above. Strain 1100-SC38 is a mutant that produces significantly more ofthe avermectin cyclohexyl-B2 form compared to avermectin cyclohexyl-B1form when supplemented with cyclohexane carboxylic acid (B2:B1 of about30:1). pSE119 used to transform S. avermitilis protoplasts was isolatedfrom either E. coli strain GM2163 (obtained from Dr. B. J. Bachmann,Curator, E. coli Genetic Stock Center, Yale University), E. coli strainDM1 (BRL), or S. lividans strain TK64. Thiostrepton resistanttransformants of strain 1100-SC38 were isolated and analyzed by HPLCanalysis of fermentation products. Transformants of S. avermitilisstrain 1100-SC38 containing pSE119 produced an altered ratio ofavermectin cyclohexyl-B2:cyclohexyl-B1 of about 3.7:1 (TABLE 2).

Having established that pSE119 was able to modulate avermectin B2:B1ratios in an AveC mutant, the insert DNA was sequenced. Approximately 10μg of pSE119 were isolated using a plasmid DNA isolation kit (Qiagen,Valencia, Calif.) following manufacturer's instructions, and sequencedusing an ABI 373A Automated DNA Sequencer (Perkin Elmer, Foster City,Calif.). Sequence data was assembled and edited using Genetic ComputerGroup programs (GCG, Madison, Wis.). The DNA sequence and the aveC ORFare presented in FIG. 1 (SEQ ID NO:1).

A new plasmid, designated as pSE118, was constructed as follows.Approximately 5 μg of pSE66 was digested with SphI and BamHI. Thereaction mixture was loaded on a 0.8% SeaPlaque GTG agarose gel (FMCBioProducts), a 2.8 Kb SphI/BamHI fragment was excised from the gelafter electrophoresis, and the DNA was recovered from the gel usingGELase™ (Epicentre Technologies) using the Fast Protocol. Approximately5 μg of the shuttle vector pWHM3 was digested with SphI and BamHI. About0.5 μg of the 2.8 Kb insert and 0.5 μg of digested pWHM3 were mixedtogether and incubated overnight with 1 unit of ligase (New EnglandBiolabs) at 15° C. in a total volume of 20 μl according to supplier'sinstructions. After incubation, 5 μl of the ligation mixture wasincubated at 70° C. for 10 min, cooled to rm temp, and used to transformcompetent E. coli DH5α cells according to manufacturer's instructions.Plasmid DNA was isolated from ampicillin resistant transformants, andthe presence of the 2.8 Kb SphI/BamHI insert was confirmed byrestriction analysis. This plasmid was designated as pSE118. The insertDNA in pSE118 and pSE119 overlap by approximately 838 nucleotides (FIG.4).

Protoplasts of S. avermitilis strain 1100-SC38 were transformed withpSE118 as above. Thiostrepton resistant transformants of strain1100-SC38 were isolated and analyzed by HPLC analysis of fermentationproducts. Transformants of S. avermitilis strain 1100-SC38 containingpSE118 were not altered in the ratios of avermectin cyclohexyl-B2:avermectin cyclohexyl-B1 compared to strain 1100-SC38 (TABLE 2).

7.1.10. PCR Amplification of the aveC Gene from S. avermitilisChromosomal DNA

A ˜1.2 Kb fragment containing the aveC ORF was isolated from S.avermitilis chromosomal DNA by PCR amplification using primers designedon the basis of the aveC nucleotide sequence obtained above. The PCRprimers were supplied by Genosys Biotechnologies, Inc. (Texas). Therightward primer was: 5′-TCACGAAACCGGACACAC-3′ (SEQ ID NO:6); and theleftward primer was: 5′-CATGATCGCTGAACCGAG-3′ (SEQ ID NO:7). The PCRreaction was carried out with Deep Vent™ polymerase (New EnglandBiolabs) in buffer provided by the manufacturer, and in the presence of300 μM dNTP, 10% glycerol, 200 pmol of each primer, 0.1 μg template, and2.5 units enzyme in a final volume of 100 μl, using a Perkin-Elmer Cetusthermal cycler. The thermal profile of the first cycle was 95° C. for 5min (denaturation step), 60° C. for 2 min (annealing step), and 72° C.for 2 min (extension step). The subsequent 24 cycles had a similarthermal profile except that the denaturation step was shortened to 45sec and the annealing step was shortened to 1 min.

The PCR product was electrophoresed in a 1% agarose gel and a single DNAband of ˜1.2 Kb was detected. This DNA was purified from the gel, andligated with 25 ng of linearized, blunt pCR-Blunt vector (Invitrogen) ina 1:10 molar vector-to-insert ratio following manufacturer'sinstructions. The ligation mixture was used to transform One Shot™Competent E. coli cells (Invitrogen) following manufacturer'sinstructions. Plasmid DNA was isolated from ampicillin resistanttransformants, and the presence of the ˜1.2 Kb insert was confirmed byrestriction analysis. This plasmid was designated as pSE179.

The insert DNA from pSE179 was isolated by digestion with BamHI/XbaI,separated by electrophoresis, purified from the gel, and ligated withshuttle vector pWHM3, which had also been digested with BamHI/XbaI, in atotal DNA concentration of 1 μg in a 1:5 molar vector-to-insert ratio.The ligation mixture was used to transform competent E. coli DH5α cellsaccording to manufacturer's instructions. Plasmid DNA was isolated fromampicillin resistant transformants and the presence of the ˜1.2 Kbinsert was confirmed by restriction analysis. This plasmid, which wasdesignated as pSE186 (FIG. 2, ATCC 209604), was transformed into E. coliDM1, and plasmid DNA was isolated from ampicillin resistanttransformants.

7.2. Results

A 2.9 Kb SacI/BamHI fragment from pSE119 was identified that, whentransformed into S. avermitilis strain 1100-SC38, significantly alteredthe ratio of B2:B1 avermectin production. S. avermitilis strain1100-SC38 normally has a B2:B1 ratio of about 30:1, but when transformedwith a vector comprising the 2.9 Kb SacI/BamHI fragment, the ratio ofB2:B1 avermectin decreased to about 3.7:1. Post-fermentation analysis oftransformant cultures verified the presence of the transforming DNA.

The 2.9 Kb pSE119 fragment was sequenced and a ˜0.9 Kb ORF wasidentified (FIG. 1) (SEQ ID NO:1), which encompasses a PstI/SphIfragment that had previously been mutated elsewhere to produce B2products only (Ikeda et al., 1995, above). A comparison of this ORF, orits corresponding deduced polypeptide, against known databases (GenEMBL,SWISS-PROT) did not show any strong homology with known DNA or proteinsequences.

TABLE 2 presents the fermentation analysis of S. avermitilis strain1100-SC38 transformed with various plasmids.

TABLE 2 Avg. S. avermitilis strain No. Transformants B2:B1 (transformingplasmid) Tested Ratio 1100-SC38 (none)  9 30.66 1100-SC38 (pWHM3) 2131.3 1100-SC38 (pSE119) 12 3.7 1100-SC38 (pSE118) 12 30.4 1100-SC38(pSE185) 14 27.9

8. EXAMPLE Construction of S. Avermitilis AveC Mutants

This example describes the construction of several different S.avermitilis AveC mutants using the compositions and methods describedabove. A general description of techniques for introducing mutationsinto a gene in Streptomyces is described by Kieser and Hopwood, 1991,Meth. Enzym. 204:430-458. A more detailed description is provided byAnzai et al., 1988, J. Antibiot. XLI(2):226-233, and by Stutzman-Engwallet al., 1992, J. Bacteriol. 174(1):144-154. These references areincorporated herein by reference in their entirety.

8.1. Inactivation of the S. avermitilis aveC Gene

AveC mutants containing inactivated aveC genes were constructed usingseveral methods, as detailed below.

In the first method, a 640 bp SphIlPstI fragment internal to the aveCgene in pSE119 (plasmid described in Section 7.1.9, above) was replacedwith the ermE gene (for erythromycin resistance) from Sacc. erythraea.The ermE gene was isolated from plJ4026 (provided by the John InnesInstitute, Norwich, U.K.; see also Bibb et al., 1985, Gene 41:357-368)by restriction enzyme digestion with Bg/II and EcoRI, followed byelectrophoresis, and was purified from the gel. This ˜1.7 Kb fragmentwas ligated into pGEM7Zf (Promega) which had been digested with BamHIand EcoRI, and the ligation mixture transformed into competent E. coliDH5α cells following manufacturer's instructions. Plasmid DNA wasisolated from ampicillin resistant transformants, and the presence ofthe ˜1.7 Kb insert was confirmed by restriction analysis. This plasmidwas designated as pSE27.

pSE118 (described in Section 7.1.9, above) was digested with SphI andBamHI, the digest electrophoresed, and the ˜2.8 Kb SphI/BamHI insertpurified from the gel. pSE119 was digested with PstI and EcoRI, thedigest electrophoresed, and the ˜1.5 Kb PstI/EcoRI insert purified fromthe gel. Shuttle vector pWHM3 was digested with BamHI and EcoRI. pSE27was digested with PstI and SphI, the digest electrophoresed, and the˜1.7 Kb PstI/SphI insert purified from the gel. All four fragments(i.e., ˜2.8 Kb, ˜1.5 Kb, ˜7.2 Kb, ˜1.7 Kb) were ligated together in a4-way ligation. The ligation mixture was transformed into competent E.coli DH5α cells following manufacturer's instructions. Plasmid DNA wasisolated from ampicillin resistant transformants, and the presence ofthe correct insert was confirmed by restriction analysis. This plasmidwas designated as pSE180 (FIG. 3; ATCC 209605).

pSE180 was transformed into S. lividans TK64 and transformed coloniesidentified by resistance to thiostrepton and erythromycin. pSE180 wasisolated from S. lividans and used to transform S. avermitilisprotoplasts. Four thiostrepton resistant S. avermitilis transformantswere identified, and protoplasts were prepared and plated undernon-selective conditions on RM14 media. After the protoplasts hadregenerated, single colonies were screened for the presence oferythromycin resistance and the absence of thiostrepton resistance,indicating chromosomal integration of the inactivated aveC gene and lossof the free replicon. One Erm^(r) Thio^(s) transformant was identifiedand designated as strain SE180-11. Total chromosomal DNA was isolatedfrom strain SE180-11, digested with restriction enzymes BamHI, HindIII,PstI, or SphI, resolved by electrophoresis on a 0.8% agarose gel,transferred to nylon membranes, and hybridized to the ermE probe. Theseanalyses showed that chromosomal integration of the ermE resistancegene, and concomitant deletion of the 640 bp PstI/SphI fragment hadoccurred by a double crossover event. HPLC analysis of fermentationproducts of strain SE180-11 showed that normal avermectins were nolonger produced (FIG. 5A).

In a second method for inactivating the aveC gene, the 1.7 Kb ermE genewas removed from the chromosome of S. avermitilis strain SE180-11,leaving a 640 bp PstI/SphI deletion in the aveC gene. A gene replacementplasmid was constructed as follows: pSE180 was partially digested withXbaI and an ˜11.4 Kb fragment purified from the gel. The ˜11.4 Kb bandlacks the 1.7 Kb ermE resistance gene. The DNA was then ligated andtransformed into E. coli DH5α cells. Plasmid DNA was isolated fromampicillin resistant transformants and the presence of the correctinsert was confirmed by restriction analysis. This plasmid, which wasdesignated as pSE184, was transformed into E. coli DM1, and plasmid DNAisolated from ampicillin resistant transformants. This plasmid was usedto transform protoplasts of S. avermitilis strain SE180-11. Protoplastswere prepared from thiostrepton resistant transformants of strainSE180-11 and were plated as single colonies on RM14. After theprotoplasts had regenerated, single colonies were screened for theabsence of both erythromycin resistance and thiostrepton resistance,indicating chromosomal integration of the inactivated aveC gene and lossof the free replicon containing the ermE gene. One Erm^(s) Thio^(s)transformant was identified and designated as SE184-1-13. Fermentationanalysis of SE184-1-13 showed that normal avermectins were not producedand that SE184-1-13 had the same fermentation profile as SE180-11.

In a third method for inactivating the aveC gene, a frameshift wasintroduced into the chromosomal aveC gene by adding two G's after the Cat nt position 471 using PCR, thereby creating a BspE1 site. Thepresence of the engineered BspE1 site was useful in detecting the genereplacement event. The PCR primers were designed to introduce aframeshift mutation into the aveC gene, and were supplied by GenosysBiotechnologies, Inc. The rightward primer was:5′-GGTTCCGGATGCCGTTCTCG-3′ (SEQ ID NO:8) and the leftward primer was:5′-AACTCCGGTCGACTCCCCTTC-3′ (SEQ ID NO:9). The PCR conditions were asdescribed in Section 7.1.10 above. The 666 bp PCR product was digestedwith SphI to give two fragments of 278 bp and 388 bp, respectively. The388 bp fragment was purified from the gel.

The gene replacement plasmid was constructed as follows: shuttle vectorpWHM3 was digested with EcoRI and BamHI. pSE119 was digested with BamHIand SphI, the digest electrophoresed, and a ˜840 bp fragment waspurified from the gel. pSE119 was digested with EcoRI and XmnI, thedigest was resolved by electrophoresis, and a ˜1.7 Kb fragment waspurified from the gel. All four fragments (ie., ˜7.2 Kb, ˜840 bp, ˜1.7Kb, and 388 bp) were ligated together in a 4-way ligation. The ligationmixture was transformed into competent E. coli DH5α cells. Plasmid DNAwas isolated from ampicillin resistant transformants and the presence ofthe correct insert was confirmed by restriction analysis and DNAsequence analysis. This plasmid, which was designated as pSE185, wastransformed into E. coli DM1 and plasmid DNA isolated from ampicillinresistant transformants. This plasmid was used to transform protoplastsof S. avermitilis strain 1100-SC38. Thiostrepton resistant transformantsof strain 1100-SC38 were isolated and analyzed by HPLC analysis offermentation products.

pSE185 did not significantly alter the B2:B1 avermectin ratios whentransformed into S. avermitilis strain 1100-SC38 (TABLE 2).

pSE185 was used to transform protoplasts of S. avermitilis to generate aframeshift mutation in the chromosomal aveC gene. Protoplasts wereprepared from thiostrepton resistant transformants and plated as singlecolonies on RM14. After the protoplasts had regenerated, single colonieswere screened for the absence of thiostrepton resistance. ChromosomalDNA from thiostrepton sensitive colonies was isolated and screened byPCR for the presence of the frameshift mutation integrated into thechromosome. The PCR primers were designed based on the aveC nucleotidesequence, and were supplied by Genosys Biotechnologies, Inc. (Texas).The rightward PCR primer was: 5′-GCMGGATACGGGGACTAC-3′ (SEQ ID NO:10)and the leftward PCR primer was: 5′-GAACCGACCGCCTGATAC-3′ (SEQ IDNO:11), and the PCR conditions were as described in Section 7.1.10above. The PCR product obtained was 543 bp and, when digested withBspE1, three fragments of 368 bp, 96 bp, and 79 bp were observed,indicating chromosomal integration of the inactivated aveC gene and lossof the free replicon.

Fermentation analysis of S. avermitilis mutants containing theframeshift mutation in the aveC gene showed that normal avermectins wereno longer produced, and that these mutants had the same fermentationHPLC profile as strains SE180-11 and SE184-1-13. One Thio^(s)transformant was identified and designated as strain SE185-5a.

Additionally, a mutation in the aveC gene that changes nt position 520from G to A, which results in changing the codon encoding a tryptophan(W) at position 116 to a termination codon, was produced. An S.avermitilis strain with this mutation did not produce normal avermectinsand had the same fermentation profile as strains SE180-11, SE184-1-13,and SE185-5a.

Additionally, mutations in the aveC gene that change both: (i) ntposition 970 from G to A, which changes the amino acid at position 266from a glycine (G) to an aspartate (D), and (ii) nt position 996 from Tto C, which changes the amino acid at position 275 from tyrosine (Y) tohistidine (H), were produced. An S. avermitilis strain with thesemutations (G266D/Y275H) did not produce normal avermectins and had thesame fermentation profile as strains SE180-11, SE184-1-13, and SE185-5a.

The S. avermitilis aveC inactivation mutant strains SE180-11,SE184-1-13, SE185-5a, and others provided herewith, provide screeningtools to assess the impact of other mutations in the aveC gene. pSE186,which contains a wild-type copy of the aveC gene, was transformed intoE. coli DM1, and plasmid DNA was isolated from ampicillin resistanttransformants. This pSE186 DNA was used to transform protoplasts of S.avermitilis strain SE180-11. Thiostrepton resistant transformants ofstrain SE180-11 were isolated, the presence of erythromycin resistancewas determined, and Thio^(r) Erm^(r) transformants were analyzed by HPLCanalysis of fermentation products. The presence of the functional aveCgene in trans was able to restore normal avermectin production to strainSE180-11 (FIG. 5B).

8.2. Analysis of Mutations in the aveC Gene That Alter Class B2:B1Ratios

As described above, S. avermitilis strain SE180-11 containing aninactive aveC gene was complemented by transformation with a plasmidcontaining a functional aveC gene (pSE186). Strain SE180-11 was alsoutilized as a host strain to characterize other mutations in the aveCgene, as described below.

Chromosomal DNA was isolated from strain 1100-SC38, and used as atemplate for PCR amplification of the aveC gene. The 1.2 Kb ORF wasisolated by PCR amplification using primers designed on the basis of theaveC nucleotide sequence. The rightward primer was SEQ ID NO:6 and theleftward primer was SEQ ID NO:7 (see Section 7.1.10, above). The PCR andsubcloning conditions were as described in Section 7.1.10. DNA sequenceanalysis of the 1.2 Kb ORF shows a mutation in the aveC gene thatchanges nt position 337 from C to T, which changes the amino acid atposition 55 from serine (S) to phenylalanine (F). The aveC genecontaining the S55F mutation was subcloned into pWHM3 to produce aplasmid which was designated as pSE187, and which was used to transformprotoplasts of S. avermitilis strain SE180-11. Thiostrepton resistanttransformants of strain SE180-11 were isolated, the presence oferythromycin resistance was determined, and Thio^(r) Erm^(r)transformants were analyzed by HPLC analysis of fermentation products.The presence of the aveC gene encoding a change at amino acid residue 55(S55F) was able to restore normal avermectin production to strainSE180-11 (FIG. 5C); however, the cyclohexyl B2:cyclohexyl B1 ratio wasabout 26:1, as compared to strain SE180-11 transformed with pSE186,which had a ratio of B2:B1 of about 1.6:1 (TABLE 3), indicating that thesingle mutation (S55F) modulates the amount of cyclohexyl-B2 producedrelative to cyclohexyl-B1.

Another mutation in the aveC gene was identified that changes ntposition 862 from G to A, which changes the amino acid at position 230from glycine (G) to aspartate (D). An S. avermitilis strain having thismutation (G230D) produces avermectins at a B2:B1 ratio of about 30:1.

8.3. Mutations That Reduce the B2:B1 Ratio

Several mutations were constructed that reduce the amount ofcyclohexyl-B2 produced relative to cyclohexyl-B1, as follows.

A mutation in the aveC gene was identified that changes nt position 588from G to A, which changes the amino acid at position 139 from alanine(A) to threonine (T). The aveC gene containing the A139T mutation wassubcloned into pWHM3 to produce a plasmid which was designated pSE188,and which was used to transform protoplasts of S. avermitilis strainSE180-11. Thiostrepton resistant transformants of strain SE180-11 wereisolated, the presence of erythromycin resistance was determined, andThio^(r) Erm^(r) transformants were analyzed by HPLC analysis offermentation products. The presence of the mutated aveC gene encoding achange at amino acid residue 139 (A139T) was able to restore avermectinproduction to strain SE180-11 (FIG. 5D); however, the B2:B1 ratio wasabout 0.94:1, indicating that this mutation reduces the amount ofcyclohexyl-B2 produced relative to cyclohexyl-B1. This result wasunexpected because published results, as well as the results ofmutations described above, have only demonstrated either inactivation ofthe aveC gene or increased production of the B2 form of avermectinrelative to the B1 form (TABLE 3).

Because the A139T mutation altered the B2:B1 ratios in the morefavorable B1 direction, a mutation was constructed that encoded athreonine instead of a serine at amino acid position 138. Thus, pSE186was digested with EcoRI and cloned into pGEM3Zf (Promega) which had beendigested with EcoRI. This plasmid, which was designated as pSE186a, wasdigested with ApaI and KpnI, the DNA fragments separated on an agarosegel, and two fragments of ˜3.8 Kb and ˜0.4 Kb were purified from thegel. The ˜1.2 Kb insert DNA from pSE186 was used as a PCR template tointroduce a single base change at nt position 585. The PCR primers weredesigned to introduce a mutation at nt position 585, and were suppliedby Genosys Biotechnologies, Inc. (Texas). The rightward PCR primer was:5′-GGGGGCGGGCCCGGGTGCGGAGGCGGAAATGCCCCTGGCGACG-3′ (SEQ ID NO:12); andthe leftward PCR primer was: 5′-GGAACCGACCGCCTGATACA-3′ (SEQ ID NO:13).The PCR reaction was carried out using an Advantage GC genomic PCR kit(Clonetech Laboratories, Palo Alto, Calif.) in buffer provided by themanufacturer in the presence of 200 μM dNTPs, 200 pmol of each primer,50 ng template DNA, 1.0 M GC-Melt and 1 unit KlenTaq Polymerase Mix in afinal volume of 50 μl. The thermal profile of the first cycle was 94° C.for 1 min; followed by 25 cycles of 94° C. for 30 sec and 68° C. for 2min; and 1 cycle at 68° C. for 3 min. A PCR product of 295 bp wasdigested with ApaI and KpnI to release a 254 bp fragment, which wasresolved by electrophoresis and purified from the gel. All threefragments (˜3.8 Kb, ˜0.4 Kb and 254 bp) were ligated together in a 3-wayligation. The ligation mixture was transformed into competent E. coliDH5α cells. Plasmid DNA was isolated from ampicillin resistanttransformants, and the presence of the correct insert was confirmed byrestriction analysis. This plasmid was designated as pSE198.

pSE198 was digested with EcoRI, cloned into pWHM3, which had beendigested with EcoRI, and transformed into E. coli DH5α cells. PlasmidDNA was isolated from ampicillin resistant transformants and thepresence of the correct insert was confirmed by restriction analysis andDNA sequence analysis. This plasmid DNA was transformed into E. coliDM1, plasmid DNA was isolated from ampicillin resistant transformants,and the presence of the correct insert was confirmed by restrictionanalysis. This plasmid, which was designated as pSE199, was used totransform protoplasts of S. avermitilis strain SE180-11. Thiostreptonresistant transformants of strain SE180-11 were isolated, the presenceof erythromycin resistance was determined, and Thio^(r) Erm^(r)transformants were analyzed by HPLC analysis of fermentation products.The presence of the mutated aveC gene encoding a change at amino acidresidue 138 (S138T) was able to restore normal avermectin production tostrain SE180-11; however, the B2:B1 ratio was 0.88:1 indicating thatthis mutation reduces the amount of cyclohexyl-B2 produced relative tocyclohexyl-B1 (TABLE 3). This B2:B1 ratio is even lower than the 0.94:1ratio observed with the A139T mutation produced by transformation ofstrain SE180-11 with pSE188, as described above.

Another mutation was constructed to introduce a threonine at both aminoacid positions 138 and 139. The ˜1.2 Kb insert DNA from pSE186 was usedas a PCR template. The PCR primers were designed to introduce mutationsat nt positions 585 and 588, and were supplied by GenosysBiotechnologies, Inc. (Texas). The rightward PCR primer was:5′-GGGGGCGGGCCCGGGTGCGGAGGCGGAAATGCCGCTGGCGACGACC-3′ (SEQ ID NO:14); andthe leftward PCR primer was: 5′-GGAACATCACGGCATTCACC-3′ (SEQ ID NO:15).The PCR reaction was performed using the conditions describedimmediately above in this Section. A PCR product of 449 bp was digestedwith ApaI and KpnI to release a 254 bp fragment, which was resolved byelectrophoresis and purified from the gel. pSE186a was digested withApaI and KpnI, the DNA fragments separated on an agarose gel, and twofragments of −3.8 Kb and ˜0.4 Kb were purified from the gel. All threefragments (˜3.8 Kb, ˜0.4 Kb and 254 bp) were ligated together in a 3-wayligation, and the ligation mixture was transformed into competent E.coli DH5α cells. Plasmid DNA was isolated from ampicillin resistanttransformants, and the presence of the correct insert was confirmed byrestriction analysis. This plasmid was designated as pSE230.

pSE230 was digested with EcoRI, cloned into pWHM3, which had beendigested with EcoRI, and transformed into E. coli DH5α cells. PlasmidDNA was isolated from ampicillin resistant transformants and thepresence of the correct insert was confirmed by restriction analysis andDNA sequence analysis. This plasmid DNA was transformed into E. coliDM1, plasmid DNA isolated from ampicillin resistant transformants, andthe presence of the correct insert was confirmed by restrictionanalysis. This plasmid, which was designated as pSE231, was used totransform protoplasts of S. avermitilis strain SE180-11. Thiostreptonresistant transformants of SE180-11 were isolated, the presence oferythromycin resistance was determined, and Thio^(r) Erm^(r)transformants were analyzed by fermentation. The presence of the doublemutated aveC gene, encoding S138T/A139T, was able to restore normalavermectin production to strain SE180-11; however, the B2:B1 ratio was0.84:1 showing that this mutation further reduces the amount ofcyclohexyl-B2 produced relative to cyclohexyl-B1 (TABLE 3), over thereductions provided by transformation of strain SE180-11 with pSE188 orpSE199, as described above.

Another mutation was constructed to further reduce the amount ofcyclohexyl-B2 produced relative to cyclohexyl-B1. Because theS138T/A139T mutations altered the B2:B1 ratios in the more favorable B1direction, a mutation was constructed to introduce a threonine at aminoacid position 138 and a phenylalanine at amino acid position 139. The˜1.2 Kb insert DNA from pSE186 was used as a PCR template. The PCRprimers were designed to introduce mutations at nt positions 585(changing a T to A), 588 (changing a G to T), and 589 (changing a C toT), and were supplied by Genosys Biotechnologies, Inc. (Texas). Therightward PCR primer was: 5′-GGGGGCGGGCCCGGGTGCGGAGGCGGAAATGCCGCTGGCGACGTTC-3′ (SEQ ID NO:16); and the leftward PCR primer was:5′-GGAACATCACGGCATTCACC-3′ (SEQ ID NO:15). The PCR reaction was carriedout using an Advantage GC genomic PCR kit (Clonetech Laboratories, PaloAlto, Calif.) in buffer provided by the manufacturer in the presence of200 μM dNTPs, 200 pmol of each primer, 50 ng template DNA, 1.1 mM Mgacetate, 1.0 M GC-Melt and 1 unit Tth DNA Polymerase in a final volumeof 50 pl. The thermal profile of the first cycle was 94° C. for 1 min;followed by 25 cycles of 94° C. for 30 sec and 68° C. for 2 min; and 1cycle at 68° C. for 3 min. A PCR product of 449 bp was digested withApaI and KpnI to release a 254 bp fragment, which was resolved byelectrophoresis and purified from the gel. All three fragments (˜3.8 Kb,˜0.4 Kb and 254 bp) were ligated together in a 3-way ligation. Theligation mixture was transformed into competent E. coli DH5α cells.Plasmid DNA was isolated from ampicillin resistant transformants, andthe presence of the correct insert was confirmed by restrictionanalysis. This plasmid was designated as pSE238.

pSE238 was digested with EcoRI, cloned into pWHM3, which had beendigested with EcoRI, and transformed into E. coli DH5α cells. PlasmidDNA was isolated from ampicillin resistant transformants and thepresence of the correct insert was confirmed by restriction analysis andDNA sequence analysis. This plasmid DNA was transformed into E. coliDM1, plasmid DNA was isolated from ampicillin resistant transformants,and the presence of the correct insert was confirmed by restrictionanalysis. This plasmid, which was designated as pSE239, was used totransform protoplasts of S. avermitilis strain SE180-11. Thiostreptonresistant transformants of strain SE180-11 were isolated, the presenceof erythromycin resistance was determined, and Thio^(r) Erm^(r)transformants were analyzed by HPLC analysis of fermentation products.The presence of the double mutated aveC gene encoding S138T/A139F wasable to restore normal avermectin production to strain SE180-11;however, the B2:B1 ratio was 0.75:1 showing that this mutation furtherreduced the amount of cyclohexyl-B2 produced relative to cyclohexyl-B1(TABLE 3) over the reductions provided by transformation of strainSE180-11 with pSE188, pSE199, or pSE231 as described above.

TABLE 3 No. Avg. S. avermitilis strain Transformants Relative RelativeB2:B1 (transforming plasmid) Tested B2 Conc. B1 Conc. Ratio SE180-11(none) 30  0  0 0 SE180-11 (pWHM3) 30  0  0 0 SE180-11 (pSE186) 26 222140 1.59 SE180-11 (pSE187) 12 283  11 26.3 SE180-11 (pSE188) 24 193 2060.94 SE180-11 (pSE199) 18 155 171 0.88 SE180-11 (pSE231)  6 259 309 0.84SE180-11 (pSE239) 20 184 242 0.75

Additional mutations were constructed to further reduce the amount ofcyclohexyl-B2 produced relative to cyclohexyl-B1 using the technique ofDNA shuffling as described in Stemmer, 1994, Nature 370:389-391; andStemmer, 1994, Proc. Natl. Acad. Sci. USA 91:10747-10751; and furtherdescribed in U.S. Pat. Nos. 5,605,793; 5,811,238; 5,830,721; and5,837,458.

DNA shuffled plasmids containing mutated aveC genes were transformedinto competent dam dcm E. coli cells. Plasmid DNA was isolated fromampicillin resistant transformants, and used to transform protoplasts ofS. avermitilis strain SE180-11. Thiostrepton resistant transformants ofstrain SE180-11 were isolated and screened for the production ofavermectins with a cyclohexyl-B2:cyclohexyl-B1 ratio of 1:1 or less. TheDNA sequence of plasmid DNA from SE180-11 transformants producingavermectins with a B2:B1 ratio of 1:1 or less was determined.

Eight transformants were identified that produced reduced amounts ofcyclohexyl-B2 relative to cyclohexyl-B1. The lowest B2:B1 ratio achievedamong these transformants was 040:1 (TABLE 4). Plasmid DNA was isolatedfrom each of the eight transformants and the DNA sequence determined toidentify the mutations in the aveC gene. The mutations are as follows.

pSE290 contains 4 nucleotide mutations at nt position 317 from T to A,at nt position 353 from C to A, at nt position 438 from G to A, and atnt position 1155 from T to A. The nucleotide change at nt position 317changes the amino acid at AA position 48 from D to E and the nucleotidechange at nt position 438 changes the amino acid at AA position 89 fromA to T. The B2:B1 ratio produced by cells carrying this plasmid was0.42:1 (TABLE 4).

pSE291 contains 4 nucleotide mutations at nt position 272 from G to A,at nt position 585 from T to A, at nt position 588 from G to A, and atnt position 708 from G to A. The nucleotide change at nt position 585changes the amino acid at AA position 138 from S to T, the nucleotidechange at nt position 588 changes the amino acid at AA position 139 fromA to T, and the nucleotide change at nt position 708 changes the aminoacid at AA position 179 from G to S. The B2:B1 ratio produced by cellscarrying this plasmid was 0.57:1 (TABLE 4).

pSE292 contains the same four nucleotide mutations as pSE290. The B2:B1ratio produced by cells carrying this plasmid was 0.40:1 (TABLE 4).

pSE293 contains 6 nucleotide mutations at nt 24 from A to G, at ntposition 286 from A to C, at nt position 497 from T to C, at nt position554 from C to T, at nt position 580 from T to C, and at nt position 886from A to T. The nucleotide change at nt position 286 changes the aminoacid at AA position 38 from Q to P, the nucleotide change at nt position580 changes the amino acid at AA position 136 from L to P, and thenucleotide change at nt position 886 changes the amino acid at AAposition 238 from E to D. The B2:B1 ratio produced by cells carryingthis plasmid was 0.68:1 (TABLE 4).

pSE294 contains 6 nucleotide mutations at nt 469 from T to C, at ntposition 585 from T to A, at nt position 588 from G to A, at nt position708 from G to A, at nt position 833 from C to T, and at nt position 1184from G to A. In addition, nts at positions 173, 174, and 175 aredeleted. The nucleotide change at nt position 469 changes the amino acidat AA position 99 from F to S, the nucleotide change at nt position 585changes the amino acid at AA position 138 from S to T, the nucleotidechange at nt position 588 changes the amino acid at AA position 139 fromA to T, and the nucleotide change at nt position 708 changes the aminoacid from AA position 179 from G to S. The B2:B1 ratio produced by cellscarrying this plasmid was 0.53:1 (TABLE 4).

pSE295 contains 2 nucleotide mutations at nt 588 from G to A and at nt856 from T to C. The nucleotide change at nt position 588 changes theamino acid at AA position 139 from A to T and the nucleotide change atnt position 856 changes the amino acid at AA position 228 from M to T.The B2:B1 ratio produced by cells carrying this plasmid was 0.80:1(TABLE 4).

pSE296 contains 5 nucleotide mutations at nt position 155 from T to C,at nt position 505 from G to T, at nt position 1039 from C to T, at ntposition 1202 from C to T, and at nt position 1210 from T to C. Thenucleotide change at nt position 505 changes the amino acid at AAposition 111 from G to V and the nucleotide change at nt position 1039changes the amino acid at AA position 289 from P to L. The B2:B1 ratioproduced by cells carrying this plasmid was 0.73:1 (TABLE 4).

pSE297 contains 4 nucleotide mutations at nt position 377 from G to T,at nt position 588 from G to A, at nt position 633 from A to G, and atnt position 1067 from A to T. The nucleotide change at nt position 588changes the amino acid at AA position 139 from A to T, the nucleotidechange at nt position 633 changes the amino acid at AA position 154 fromK to E, and the nucleotide change at nt position 1067 changes the aminoacid at AA position 298 from Q to H. The B2:B1 ratio produced by cellscarrying this plasmid was 0.67:1 (TABLE 4).

TABLE 4 No. Avg. S. avermitilis strain Transformants Relative RelativeB2:B1 (transforming plasmid) Tested B2 Conc. B1 Conc. Ratio SE180-11(none) 4  0  0 0   SE180-11 (pWHM3) 4  0  0 0   SE180-11 (pSE290) 4  87208 0.42 SE180-11 (pSE291) 4 106 185 0.57 SE180-11 (pSE292) 4  91 2310.40 SE180-11 (pSE293) 4 123 180 0.68 SE180-11 (pSE294) 4  68 129 0.53SE180-11 (pSE295) 4 217 271 0.80 SE180-11 (pSE296) 1 135 186 0.73SE180-11 (pSE297) 1 148 221 0.67

Additional rounds of DNA shuffling were conducted to further reduce theamount of cyclohexyl-B2 avermectin produced relative to cyclohexyl-B1avermectin as follows.

Semi-Synthetic Shuffling

The best clone was shuffled using the method described in PCTInternational Publication WO 97/20078 by Maxygen Inc. Individualoligonucleotides encoding beneficial substitutions best corresponding todecreased B2:B1 ratio were added to the shuffling reaction at 5 molarexcess over the aveC template strands. Each nucleotide mismatch of theoligonucleotide was flanked by 20 nucleotides of perfect identity toensure proper incorporation during the shuffling reaction.Oligonucleotides were purchased from Operon Technologies (Alameda,Calif.).

HTP Growth of S. avermitilis

Independent clones were picked from the transformation plates andinoculated into 200 μl R5 medium (Kieser, T., et al., “PracticalStreptomyces Genetics”, 2000, Norwich, U.K., John Innes Foundation) indeep 96-well seed plates and grown at 30° C. with shaking. In each well,a glass-bead was dispensed for dispersion of mycelia and agitation ofthe culture. During this time, the cultures attained even and densegrowth. After 4-5 days, 20 μl of the seed medium culture was dispensedto production plates and the remaining volume was frozen as masterplates at −80° C. after the addition of glycerol to the finalconcentration of 20%. The production plates were incubated at 300 underhumidity for 12-14 days. Sporulation of the strains occurred after 5-8days of incubation. The production plates were made essentially asdescribed in PCT International Publication WO 99/41389 by Pfizer Inc.,with the exception of adding 1% agarose to ensure a solid surface.

Extraction and ESI-MS/MS Screening

A total of 1 ml ethyl acetate was added to each well and incubatedshaking at room temperature for 20 minutes. Approximately 750 μl of theethyl acetate-phase was recovered, transferred to a 96-well plate andset to evaporate over night. The precipitate was resuspended in 100 μlmethanol 1 mM NaCl of which 5 μl solution was injected into massspectrometer by an autosampler in a 96-well format and analyzed directlyin the flow injection phase without liquid chromatography or otherseparation. The compounds were ionized by electrospray ionization andtwo separate channels were monitored on two MS/MS transitions. The MS/MStransition for B1 sodiated ion is from m/z 921 to m/z 777 and for B2sodiated ion is from m/z 939 to m/z 795 in positive mode. A FinniganTSQ-7000, Micromass Quattro-LC mass spectrometer and a Leap TechnologyTwin-Pal autosampler were used for this high throughput screening.Integration of the separate B1 and B2 chromatograms for each welllocation identified the hits.

Eighty-eight (88) new combinations of amino acid substitutions wereidentified that can produce ratios of cyclohexyl B2:cyclohexyl B1avermectins of 0.35:1 or less (FIG. 6). Several of these new mutationscan produce ratios of cyclohexyl B2:cyclohexyl B1 avermectins of about0.30:1 or less; several can produce ratios of cyclohexyl B2:cyclohexylB1 avermectins of about 0.25:1 or less, and several can produce ratiosof cyclohexyl B2:cyclohexyl B1 avermectins of about 0.20:1 or less, andseveral can produce ratios of cyclohexyl B2:cyclohexyl B1 of about 0.1:1or less. Two (2) new mutations were identified that can produce ratiosof cyclohexyl B2:cyclohexyl B1 avermectins of 0.37 or 0.38. Eighteen(18) new mutations were identified that can produce ratios of cyclohexylB2:cyclohexyl B1 between 0.58:1 and 1.17:1.

Deposit of Biological Materials

The following plasmids were deposited with the American Type CultureCollection (ATCC) at 12301 Parklawn Drive, Rockville, Md., 20852, USA,on Jan. 29, 1998, and were assigned the following accession numbers:

Plasmid Accession No. plasmid pSE180 209605 plasmid pSE186 209604

The current address of the American Type Culture Collection is 10801University Blvd, Manassas, Va., 20110, USA.

All patents, patent applications, and publications cited above areincorporated herein by reference in their entirety.

The present invention is not to be limited in scope by the specificembodiment described herein, which are intended as single illustrationsof individual aspects of the invention, and functionally equivalentmethods and components are within the scope of the invention. Indeed,various modifications of the invention, in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appended claims.

1. An isolated polynucleotide molecule comprising a nucleotide sequencethat is otherwise the same as the Streptomyces avermitilis aveC allele,the S. avermitilis AveC gene product-encoding sequence of plasmid pSE186(ATCC 209604) or the nucleotide sequence of the AveC ORF of S.avermitilis as presented in FIG. 1 (SEQ ID NO:1), or a degeneratevariant thereof, which nucleotide encodes a functionally equivalent aveCgene product and which nucleotide sequence further comprises mutationsencoding one or more amino acid substitutions in the amino acid sequenceof SEQ ID NO:2, such that cells of S. avermitilis strain ATCC 53692 inwhich the wild-type aveC allele has been inactivated and that expressthe polynucleotide molecule comprising the mutated nucleotide sequenceproduce a class 2:1 ratio of avermetins that is reduced compared to theratio produced by cells of S. avermitilis strain ATCC 53692 that insteadexpress only the wild-type aveC allele, wherein when the class 2:1avermectins are cyclohexyl B2: cyclohexyl B1 avermectins the ratio ofclass 2:1 avermectins is 0.35:1 of less, wherein the combinations ofamino acid substitutions comprise substitutions at both D48 and G179. 2.An isolated polynucleotide molecule comprising a nucleotide sequencethat is otherwise the same as the Streptomyces avermitllis aveC allele,the S. avermitilis aveC gene product-encoding sequence of plasmid pSE186(ATCC 209604) or the nucleotide sequence of the aveC ORF of S.avermitilis as presented in FIG. 1 (SEQ ID NO:1), or a degeneratevariant thereof, which nucleotide sequence further comprises mutationsencoding one or more amino acid substitutions in the amino acid sequenceof SEQ ID NO:2, such chat cells of S. avermitilis strain ATCC 53692 inwhich the wild-type aveC allele has been inactivated and that expressthe polynucleotide molecule comprising the mutated nocleotide sequenceproduce a class 2:1 ratio of avermectins that is reduced compared to theratio produced by cells of S. avermitilis strain ATCC 53692 that insteadexpress only the wild type aveC allele, wherein when the class 2:1avermectins are cyclohexyl B2:cyclohexyl B1 avermectins the ratio ofclass 2:1 avermectins is 0.35:1 or less, wherein the combinations ofamino acid substitutions comprise substitution selected from the groupconsisting of: (a) D48E, A61T, A89T, S138T, G139T, G179S, A198G, P289L;(b) G40S, D48E, L136P, G179S, E238D; (c) D48E, L136P, R163Q, G179S; (d)D48E, L136P, R163Q, G179S, E238D; (e) D48E, L136P, R163Q, G179S, A200G,E238D; (f) D48E, L136P, G179S, E238D; (g) D48E, A61T, L136P, G179S,E238D; (h) D48E, A61T, L136P, G179S; (i) D48E, A89T, S138T, A139T,G179S; (j) D48E, A61T, L136P, G179S, A198G, P202S, E238D, P289L; (k)D48E, A61T, L136P, S138T, A139F, G179S, E238D, P289L; (l) D48E, L136P,G179S, A198G, E238D, P289L; (m) D48E, A61T, S138T, A139F, G179S, A198G,P289l; (n) D48E, L84P, G111V, S138T, A139T, G179S, A198G, P289L; (o)Y28C, D48E, A61T, A89T, A89T, S138T, A139T, G179S, E238D; (p) D48E,A61T, A107T, S108G, L136P, G179S, S192A, E238D, P289L; (q) D48E, L136P,G179S, R250W; (r) D48E, A89T, S138T, A139T, R163Q, G179S; (s) D48E,L136P, G179S, A198G, P289L; (t) D48E, F78L, A89T, L136P, G179S; (u)D48E, A89T, S138T, A139T, G179S, E238D, F278L; (v) D48E, A89T, L136P,R163Q, G179S; (w) D48E, A61T, A89T, G111V, S138T, A139F, G179S, E238D,P289L; (x) D25G, D48E, A89T, l136P, S138T, A139T, V141A, I159T, R163Q,G179S; (y) D48E, A89T, S90G, L136P, R163Q, G179S, E238D; (z) D48E, A61T,A89T, G111V, S138T, A139T, G179S, E238D, P289L; (aa) D48E, A89T, S138T,A139T, G179S; (ab) D48E, L136P, R163Q, G179S, S231L; (ac) D48E, L136P,S138T, A139F, G179S, V196A, E238D; (ad) D48E, A61T, A89T, F99S, S138T,A139T, G179S, E238D; (ae) G35S, D48E, A89T, S138T, A139T, G179S, D289L;(af) D48E, A61T, A89T, S138T, A139T, G179S, V196A, E238D; (ag) D48E,A89T, G111V, S138T, A139T, G179S, A198G, E238D; (ah) S41G, D48E, A89T,L136P, G179S; (ai) D48E, A89T, l136P, R163Q, G179S, D252S; (aj) D48E,A89T, L136P, G179S, F234S; (ak) D48E, A89T, L136P, R163Q, G179S, E238D;(al) Q36R, D48E, A89T, L136P, G179S, E238D; (am) D48E, A89T, L136P,R163Q, G179S; (an) D48E, A89T, S138T, G179S; (ao) D48E, A89T, L136P,G179S, E238D; (ap) D48E, A89T, L136P, K154E, G179S, E238D; (aq) D48E,A89T, S138T, A139T, K154R, G179S, V196A, P289L; (ar) D48E, A89T, S138T,A139F, G179S, V196A, E238D; (as) D48E, A61T, A89T, L136P, G179S, V196A,A198G, P289L; (at) D48E, A61T, S138T, A139F, G179S, G196A, E238D, P289L;(au) D48E, A89T, L136P, G179S; (av) D48E, A89T, V120A, L136P, G179S;(aw) D48E, A61T, A89T, S138T, A139F, G179S, V196A, A198G, E238D; (ax)D48E, A61T, A89T, G111V, S138T, A139F, G179S, V196A, E238D; (ay) D48E,A61T, A89T, S138T, A139T, G179S, V196A, E238D, P289L; (az) D48E, A61T,A89T, L136P, S138T, A139F, G179S, A198G, E238D; (ba) D48E, A89T, S138T,A139F, G179S, A198G, V220A; (bb) D48E, A61T, A89T, S138T, A139T, G179S,V196A, E238D, R239H, D289L; (bc) D48E, A61T, A89T, L136P, G179S, P289L;(bd) D48E, A89T, S138T, A139T, G179S, V196A, E238D, D289L; (be) D48E,A61T, A89T, S138T, A139F, G179S, V196A, E238D; (da) S41G, D46E, A61T,R71L, A89T, L136M, S138T, A139T, T149S, G179S, V196A, E238D, F278L,P289L; (db) S41G, D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S,F176C, G179S, V196A, E238D, P289L; (dc) D48E, R71L, A89T, L136P, T149S,F176C, G119S, E238D, I280V; (dd) D48E, A61T, R71L, W110L, T149S, G179S,V196A, L206M, E238D, V271A, I280V; (de) D48E, A61T, R71L, A89T, L136M,S138T, A139T, T149S, G179S, V196A, E238D, H279Q, P289L; (df) D48E, A61T,R71L, A89T, L136M, S138T, A139T, T149S, G179S, V196A, E238D, G287E,P289Q; (dg) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,V196A, E238D, P289L; (dh) D48E, A61T, R71L, A89T, A139T, T149S, F176C,G179S, V196A, E238D, V285G, D289L; (di) Q38R, D48E, A61T, R71L, L87V,A89T, L136M, S138T, A139T, T149S, G179S, V196A, E238D, P289L; (dj) D48E,A61T, L87V, A89T, W110L, S138T, A139T, T149S, G179S, V196A, E238D,D289L; (dk) D48E, A61T, R71L, A89T, L136M, S138T, A139T, T149S, G179S,V196A, E238D, D289L; (dl) D48E, A89T, L136P, K154E, G179S, S231L, E238D;(ea) D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S, V196A, E238D,I280V; (eb) D48E, R71L, A89T, L136P, T149S, F176C, G179S, E238D, I280V;(ec) Q36P, D48E, A61T, R71L, A89T, L136P, T149S, F176C, G179S, V196A,E238D, I280V; (ed) D48E, A61T, R71L, A89T, A139T, T149S, F176C, G179S,V196A, E238D, I280V; (ee) V2M, D48E, A61T, R71L, A89T, L136P, T149S,F176C, G179S, V196A, E238D, I280V; (ef) D48E, A61T, R71L, A89T, L136P,T149S, F176C, G179S, V196A, E238D, I280V, A302T; (eg) D48E, R71L, A89T,L136P, T149S, F176C, G179S, E238D, P289L; (eh) D48E, R71L, A89T, L136P,T149S, F176C, G179S, E238D, A302T; (ei) D48E, R71L, A89T, L136P, T149S,F176C, G179S, V196A, E238D, I280V; (ej) D48E, A61T, R71L, A89T, L136P,T149S, F176C, G179S, V196A, E238D; (ek) V2M, D48E, R71L, A89T; L136P,T149S, F176C, G179S, V196A, E238D, I280V; (el) D48E, A61T, R71L, A89T,L136P, T149S, R162H, F176C, G179S, V196A, E238D, I280V; (em) D48E, R71L,A89T, V120A, L136P, T149S, K154E, G179S, S231L, E238D; (en) D48E, R71L,A89T, V120A, L136P, T149S, F176C, G179S, S231L, E238D, I280V; (eo) D48E,A61T, R71L, L87V, A89T, A139T, T149S, F176C, G179S, V196A, E238D, V285G,D289L; (ep) D48E, A61T, R71L, L87V, A89T, S90N, A139T, T149S, F176C,G179S, V196A, E238D, V285G, D289L; (eq) D48E, R71L, A89T, L136P, K154E,G179S, S231L, E238D; (er) D48E, R71L, A89T, V120A, L136P, K154E, F176C,G179S, S231L, E238D; and (es) D48E, R71L, A89T, V120A, L136P, T149S,K154E, F176C, G179S, S231L, E238D.
 3. The polynucleotide molecule ofclaim 2, wherein the ratio of cyclohexyl B2:cyclohexyl B1 avermectins isabout 0.20:1 or less.
 4. The polynucleotide molecule of claim 2, whereinthe ratio of cyclohexyl B2:cyclohexyl B1 avermectins is about 0.10:1 orless.
 5. A recombinant vector comprising a polynucleotide molecule ofclaim
 2. 6. An isolated host cell comprising the recombinant vector ofclaim
 5. 7. The host cell of claim 6, which is a Streptomyces cell. 8.The host cell of claim 7, which is a cell of Streptomyces avermitilis.